Irreligion in Mexico refers to atheism, agnosticism, deism, religious skepticism, secular humanism, and secularism in Mexican society, which was a confessional state after independence from Imperial Spain. The first Mexican constitution was enacted in 1824, it stated that the religion of the nation is and will perpetually be the Catholic, and prohibited any other religion. Since 1857, the country has no official religion and some anti-clerical laws contained in both the 1857 and 1917 Constitutions imposed severe limitations on religious organizations and sometimes codified state intrusion into religious matters. A 1992 constitutional amendment lifted most restrictions, granting all religious groups legal status, conceding them limited property rights, granting voting rights to religious ministers and lifting restrictions on the number of priests in the country. However, religious ministers cannot be elected to public office, the government does not provide any financial contributions to religious organizations and they can not participate in public education. Although historically the Catholic Church has dominated the religious landscape of the country, according to the Catholic News Agency, there is a growing community of atheists and non-religious people,  reaching 10% according to recent polls by religious agencies. 
The Street Gazzete : Anti Clerical Manifestation depicts Mexican cavalry repressing anti-Church peasants. The cartoon by Posada satirizes the social tension between the Church and the citizens of Mexico.
When the Roman Catholic politician Vicente Fox assumed the Mexican Presidency (2000–06), liberal Mexicans feared that the official secularism of public life would be compromised.
By the year 1854, The Church gained possession of about two-thirds of all the lands of Mexico, almost every bank, and every large business. The rest of the country was mortgaged to the Church. Then came the revolution of 1854, led by Benito Juárez. It culminated in the Constitution of 1857, which secularised the schools and confiscated Church property. All the churches were nationalised, many of them were turned into schools, hospitals, and orphan asylums. Civil marriages were obligatory. Pope Pius IX immediately issued a mandate against the Constitution, and called upon all Catholics of Mexico to disobey it. Ever since then, the clergy has been fighting to regain its lost temporal power and wealth. (The Necessity of Atheism, p. 154)
At the turn of the 19th century, the collaboration of the Mexican Catholic Church with the Porfiriato, the 35-year dictatorship of General Porfirio Díaz, earned the Mexican clregy the ideological enmity of the revolutionary victors of the Mexican Revolution (1910–20); thus, the Mexican Constitution of 1917 legislated severe social and political, economic and cultural restrictions upon the Catholic Church in the Republic of Mexico. Historically, the 1917 Mexican Constitution was the first political constitution to expilicity legislate the social and civil rights of the people; and served as constitutional model for the Weimar Constitution of 1919 and the Russian Constitution of 1918. Nevertheless, like the Spanish Constitution of 1931, it has been characterized as being hostile to religion.
The Constitution of 1917 proscribed the Catholic clergy from working as teachers and as instructors in public and private schools; established State control over the internal matters of the Mexican Catholic Church; nationalized all Church property; proscribed religious orders; forbade the presence in Mexico of foreign-born priests; granted each state of the Mexican republic the power to limit the number of, and to eliminate, priests in its territory; disenfranchised priests of the civil rights to vote, and to hold elected office; banned Catholic organizations that advocated public policy; forbade religious publications from editorial commentary about public policy; proscribed the clergy from wearing clerical garb in public; and voided the right to trial of any Mexican citizen who violated anti-clerical laws.
During the Mexican Revolution, the national rancour provoked by the Church's history was aggravated by the collaboration of the High Clergy of the Mexican Catholic Church with the pro–U.S. dictatorship (1913–14) of General Victoriano Huerta, "The Usurper" of the Mexican Presidency; thus were the anti-clerical laws integral to the Mexican Constitution of 1917. In the 1920s, the enforcement of the Constitutional anti-clerical laws, by the Mexican Federal Government, provoked the Cristero Rebellion (1926–29), the clerically-abetted armed revolt of Catholic peasants, known as "The Christers" (Los cristeros). The social and political tensions between the Catholic Church and the Mexican State lessened after 1940, but the Constitutional restrictions remained the law of the land, although their enforcement became progressively lax. The Government established diplomatic relations with the Holy See during the administration of President Carlos Salinas de Gortari (1988–94), and the Government lifted almost all restrictions on the Catholic Church in 1992. That year the Government ratified its informal policy of not enforcing most legal controls on religious groups by, among other things, granting religious groups legal status, conceding them limited property rights, and lifting restrictions on the number of priests in the country. However, the law continues to mandate strict restrictions on the church and bars the clergy from holding public office, advocating partisan political views, supporting political candidates, or opposing the laws or institutions of the State. The Church's ability to own and operate mass media is also limited. Indeed, after the creation of the Constitution the Catholic Church has been acutely hostile towards the Mexican government. As Laura Randall in his book Changing Structure of Mexico points out, most of the conflicts between citizens and religious leaders lie in the Church's overwhelming lack of understanding of the role of the state's laicism. "The inability of the Mexican Catholic Episcopate to understand the modern world translates into a distorted conception of the secular world and the lay state. Evidently, perceiving the state as anti-religious (or rather, anti-clerical) is the result of 19th-century struggles that imbued the state with anti-religious and anti-clerical tinges in Latin American countries, much to the Catholic Church's chagrin. Defining laicist education as a 'secular religion' that is also 'imposed and intolerant' is the clearest evidence of episcopal intransigence." Others, however see the Mexican state's anticlericalism differently. Recent President Vicente Fox stated, "After 1917, Mexico was led by anti-Catholic Freemasons who tried to evoke the anticlerical spirit of popular indigenous President Benito Juárez of the 1880s. But the military dictators of the 1920s were a more savage lot than Juarez." Fox goes on to recount how priests were killed for trying to perform the sacraments, altars were desecrated by soldiers and freedom of religion outlawed by generals.
As many students of Latin American religion have pointed out, there is a substantial difference between describing oneself as religious or culturally religious and practising one's faith literally. In the case of Mexico the decline of religious influence of the Church is specially mirrored by the decline of church attendance among its citizens. Church attendance itself is a complex, multi-layered phenomenon that is subject to political and socio-economic factors. From 1940 to 1960 about 70% of Mexican Catholics attended church weekly while in 1982 only 54 percent partook of Mass once a week or more, and 21 percent claimed monthly attendance. Recent surveys have shown that only around 3% of Catholics attend church daily, however 47% percent of them attend church services weekly  and, according to INEGI, the number of atheists grows annually by 5.2%, while the number of Catholics grows by 1.7%.
Timeline of events related to atheism or anti-clericalism in Mexico
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1831 - Vicente Rocafuerte was arrested in Mexico for publishing an Essay on Religious Toleration. He was accused of violating Article 3 of the constitution, which stated that Mexico was a confessional state.
1844 - Ignacio Ramírez "El Nigromante" wrote "There is no God: natural beings support themselves", causing several controversies throughout the country.
1855 - The Ley Juárez (Juárez's Law) of 1855, abolished special clerical and military privileges, and declared all citizens equal before the law.
1906 - Flores Magón published his Manifesto to the Nation, The Plan of the Mexican Liberal Party declaring: "The clergy, this unrepentant traitor, this subject of Rome, this irreconcilable enemy of native liberties, in place of finding tyrants to serve and from whom to receive protection, will find instead inflexible laws which will put a limit on their excesses and which will confine them to the religious sphere."
1926 - In June 1926, Elías Calles signed the "Law for Reforming the Penal Code", known unofficially as the Calles Law. This provided specific penalties for priests and individuals who violated the provisions of the 1917 Constitution.
1926 - On November 18, 1926, the Pope issues the encyclical Iniquis Afflictisque (On the Persecution of the Church in Mexico). The Pope criticized the state's interference in matters of worship, outlawing of religious orders and the expropriation of Church property. Alluding to the deprivation of the right to vote and of free speech, among other things, he noted that, "Priests are ... deprived of all civil and political rights. They are thus placed in the same class with criminals and the insane."
1927 - November 23, 1927, Miguel Pro, SJ is killed after being convicted, without trial, on trumped-up charges of conspiring to kill President Obregon. Calles' government carefully documented execution by photograph hoping to use images to scare Cristero rebels into surrender, but the photos had the opposite effect.
1927 - September 29, 1932 Pope Pius XI issued a second encyclical on the persecution, Acerba Animi.
1934 - There were 4,500 priests serving the people before the rebellion, in 1934 there were only 334 priests licensed by the government to serve fifteen million people, the rest having been eliminated by emigration, expulsion and assassination.
1934 - Between 1926 and 1934 at least 40 priests were killed.
1935 - By 1935, 17 states had no priest at all.
1940 - Between 1931 and 1940 at least 223 rural teacher were assassinated by the Cristeros and other Catholic armed groups, because of their atheist and socialist education.
1940 - By 1940 the Church had "legally had no corporate existence, no real estate, no schools, no monasteries or convents, no foreign priests, no right to defend itself publicly or in the courts, and no hope that its legal and actual situations would improve. Its clergy were forbidden to wear clerical garb, to vote, to celebrate public religious ceremonies, and to engage in politics", but the restrictions were not always enforced.
1940 - Manuel Ávila Camacho, a professed religious believer, becomes President. This was a change from his predecessors in the first half of the 20th century who had been strongly anticlerical. His open profession of faith was politically dangerous as it risked the ire of Mexican anticlericals.
1940 - By 1940 open hostility toward the Church began to cease with the election of President Ávila (1940–46), who agreed, in exchange for the Church's efforts to maintain peace, to nonenforcement of most of the anticlerical provisions, an exception being Article 130, Section 9, which deprived the Church of the right of political speech, priests of the right to vote, and the right of free political association.
1948 - In June 1948, Diego Rivera painted the mural Dreams of a Sunday in the Alameda at the Del Prado Hotel depicting Ignacio Ramírez holding a sign reading, "God does not exist". Rivera would not remove the inscription, so the mural was not shown for 9 years – after Rivera agreed to remove the words. He stated: "To affirm 'God does not exist', I do not have to hide behind Don Ignacio Ramírez; I am an atheist and I consider religions to be a form of collective neurosis. I am not an enemy of the Catholics, as I am not an enemy of the tuberculars, the myopic or the paralytics; you cannot be an enemy of the sick, only their good friend in order to help them cure themselves." The Publicity in the newspapers had been riot-provoking, and Rivera's stand - "I will not remove one letter from it" - brought forth a mob of some thirty persons who vandalised everything in their path. They further violated the mural by defecating the self-portrait of Rivera as a young boy. On that very night, not far from the Hotel, Rivera, along with Mexico's leading artists and intellectuals, was attending a dinner honouring the director of the Museum of Fine Art. When the word arrived about the attack on Rivera's mural, it caused a stir in the audience. David Alfaro Siqueiros exhorted the guests to go to the Del Prado Hotel and, arm-in-arm with José Clemente Orozco and Dr. Atl, marched at the head of 100 people. Among them were Frida Kahlo, Juan O'Gorman, Raul Anguiano y José Revueltas. When they arrived Rivera climbed on a chair, asked for a pencil and calmly began to restore the destroyed inscription: "God does not exist".
1979 - Pope John Paul II visits Mexico and violates Mexican anticlerical laws by appearing in public wearing clerical garb and by engaging in public religious observances; some anticlericals objected to the violation of the law and President José López Portillo himself offered to pay the 50 pesos fine.
2009 - Mexico City played host to international symposium on religious freedom in Latin America sponsored by the Knights of Columbus, the first time such an event has occurred in Mexico City. Sociologist Jorge Trasloheros noted that many powerful Mexicans see religion not as "the opium of the masses", but as "the tobacco of the masses"—a bad habit to be banned from the public arena. Supreme Knight Carl Anderson denounced this idea still commonly held in Mexico that "religious beliefs are not welcome in the public square, or worse are not allowed in the public square".
2010 - In March 2010, the lower house of the Mexican legislature introduced legislation to amend the Constitution to make the Mexican government formally "laico"—meaning "lay" or "secular". Critics of the move say the "context surrounding the amendment suggests that it might be a step backwards for religious liberty and true separation of church and state". Coming on the heels of the Church's vocal objection to legalization of abortion as well as same sex unions and adoptions in Mexico City, "together with some statements of its supporters, suggests that it might be an attempt to suppress the Catholic Church's ability to engage in public policy debates". Critics of the amendment reject the idea that "Utilitarians, Nihilists, Capitalists, and Socialists can all bring their philosophy to bear on public life, but Catholics (or other religious minorities) must check their religion at the door" in a sort of "second-class citizenship" which they consider nothing more than religious discrimination.
^"Candidate Vicente Fox contributed to this perception, by sending a letter, in May 2000, to the religious authorities of various churches, in which he presented a list of ten promises, ranging from defending the right-to-life, from the moment of conception until natural death (which meant a condemnation of abortion and euthanasia), to granting religious associations access to communications media. Many of those promises were hard to keep, because no political party had an absolute majority in the Congress elected on July 6, 2000. Nonetheless, Fox's ‘Ten Promises’ were regarded, by many, as a proof of the alliance between the Catholic Church and candidate Fox." Laura Randall (2006) Page 433
^ abGerhard Robbers (2007). Encyclopedia of World Constitutions. Infobase Publishing. p. 596. ISBN [[Special:BookSources/00816060789, 9780816060788|00816060789 [[Category:Articles with invalid ISBNs]], 9780816060788]] Check |isbn= value (help).
^"I don't believe in god, but I believe in destiny." "Our working relationship involves a lot of dialogue...we have very different viewpoints on certain things, like Alejandro's Catholicism and the fact that I'm an atheist." Filter Magazine
^"Guillermo Kahlo was an educated, atheist, German-Jewish immigrant, who had come to Mexico as a young man and become an accomplished photographer, specializing in architectural photography". Samuel Brunk, Ben Fallaw, Heroes & hero cults in Latin America, (University of Texas Press, 2006), ISBN 0-292-71437-8 Page 174
^"Her father Guillermo, from whom Frida inherited her creativity, was an atheist". Patrick Marnham, Diego Rivera Dreaming with His Eyes Open: A Life of Diego Rivera, (University of California Press, 2000), ISBN 0-520-22408-6 Page 220 
^"Marcos' revolutionary weddings were breaking the Church's monopoly on matrimonial services, and the Subcommander's presiding over them was perceived by the diocese as both an encroachment on Church prerogatives and as sacrilege. Marcos and the bishop were diametrically and vehemently opposed on certain issues, in particular birth control. Marcos believed whole-heartedly in it. The guerrillas were issued contraceptive devices at a clinic in Morelia which the government had helped found and fund. Nor was the encouragement and distribution of contraceptives restricted to the guerrillas themselves. Marcos believed that one of the major contributing factors to hardship and poverty was its overpopulation. Finally, according to one source at least, Marcos was becoming increasingly intolerant regarding questions of faith, even going so far as to preach atheism" Nick Henck, Subcommander Marcos: The Man and the Mask, (Duke University Press, 2007) ISBN 0-8223-3995-1 Page 119
^The War Against Oblivion : The Zapatista Chronicles 1994–2000
^"Hasta ahora no profeso religión ni tengo razón para profesarla puesto que no creo en ninguna forma teológica". Juan O'Gorman, Autobiografía, (UNAM, 2007) ISBN 970-32-3555-7
^"God is an excuse, a foggy abstraction that everyone uses for his own benefit and moulds it to the extent of his convenience and interests". Fernando Vallejo during the ceremony of the Rómulo Gallegos Prize in Venezuela