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The term Western religion refers to religions that originated within Western culture, and are thus which historically, culturally, and theologically distinct from the Eastern religions. The contrast between Western and Eastern religions largely pertains to the distinction between monotheism and polytheism, respectively, and the term Abrahamic religion is often used in lieu of using the East and West terminology.
Western culture itself was significantly developed through the emergence of Christianity as it was introduced in the Roman Empire in the late 4th century and evolved in the course of the European Middle Ages, and the term "Christendom" largely indicates this intertwined history. Western Christianity was significantly influenced by Hellenistic religion (notably Platonism and Gnosticism) as well as the Roman imperial cult. Western Christianity is based on Roman Catholicism (Latin Rite), as opposed to Eastern Orthodoxy, from which it was divided by the Great Schism of the 11th century, and further includes all Protestant traditions splitting off Roman Catholicism from the 16th century.
"The West" as a culture or civilization historically evolved out of Greco-Roman Classical Antiquity. These cultures had polytheistic religions, viz. Greek polytheism and Roman polytheism. "Eastern" influences on these religions are evident from earliest times, the Orientalizing period at the very beginning of Greek antiquity.
During Hellenism and the Roman empire period, "Eastern" (Oriental) religions exerted a considerable influence on "Western" religion, giving rise to Persian influenced traditions like Gnosticism and Mithraism, as well as Egyptian and "Chaldean" influence on mystery religions (Orphism), astrology and magic. Early Christianity itself is a further example of Orientalizing influence on the later Roman empire.
During the same period, inherited traditions of native Roman religion were marginalized or overlaid by interpretatio graeca, and the Roman imperial cult evolved into a civil religion which involved state ritual rather than religious faith or experience. Celtic and Germanic religion was described by Roman ethnography as primitive, but at the same time as pure or unspoiled compared to the so called urban decadence of Rome.
Western Christianity 
Western Christianity is based on Roman Catholicism (Latin Rite), as opposed to Eastern Orthodoxy, from which it was divided by the Great Schism of the 11th century. Western Christianity itself was divided by the Protestant Reformation in the 16th century, and pronouncedly "Western" forms of Christianity include Puritanism and Evangelicalism, movements resulting from the various "Great Awakenings" in the 18th to 20th century Anglosphere and popularly practiced in the USA.
Renaissance magic 
Following the religious wars of the 16th to 17th centuries, the Age of Enlightenment of the 18th century paved the way for a detachment of society and politics from religious questions. Inspired by the American Revolution, the French Revolution brought the ideal of secularization and a laicist state granting freedom of religion to Europe. After the turmoils of the Napoleonic Wars, this development caught hold in other parts of Europe, by means of the German Mediatisation and the separation of church and state in numerous European constitutions drawn up after the revolutions of 1848.
New religious movements 
The principle of religious freedom introduced in Western society in the early 19th century facilitated the emergence of numerous new religious movements. Early examples were derived from western occultism and the tradition of secret societies such as the Freemasons, but from the later 19th century, influence of Eastern religions, notably Buddhism and Hinduism played an increasing role. From the mid 20th century, Eastern and Western spiritual traditions were increasingly syncretized in the various movements associated with the New Age and Neopagan countercultures.
Religion in the Western world today 
The "Western world" taken as consisting of Europe, North America and Australia-New Zealand remains predominantly Christian: 78.5% in the USA (2002), 77% in Canada (2001), close to 80% in Europe (includes Eastern Orthodoxy in Eastern Europe, not properly part of "Western religion", 32.4% of Europeans are Roman Catholic, 16.2% are Protestant) and 64% in Australia (2006).
The second largest religions in all these regions are smaller by at least an order of magnitude, Islam in Europe with about 4%, Islam in Canada with about 2%, Judaism in the United States with about 1.7%, and Islam in Australia with about 1.7%.
Most non-Christians in the Western world are irreligious, 30% in Australia, 18% in Europe, 16.1% in the USA and 16% in Canada. This is a reflection of the tradition of secular humanism which culminated in the 18th century "Age of Enlightenment" and which allowed the great progress in science and technology which accompanied the "European miracle".
There remains a minority of the order of 5% of the population in the Western world which adheres to non-Western religions, mostly due to recent immigration, but to some extent also due to proselytization, notably conversion to various sects of Buddhism and Hinduism in the context of the New Age movement in the later part of the 20th century.
See also 
- Eastern religions
- Hellenistic Judaism
- Western culture
- Religion in Europe
- Religion in the United States