Religious persecution in the Roman Empire
As the Roman Republic, and later the Roman Empire, expanded, it came to include people from a variety of cultures, and religions. The worship of an ever increasing number of deities was tolerated and accepted. The government, and the Romans in general, tended to be tolerant towards most religions and cults. Some religions were persecuted for political reasons rather than dogmatic zeal, and other rites which involved human sacrifice were banned.
In the Christian era, when Christianity became the state church of the Roman Empire, the Church came to accept it was the Emperor's duty to use secular power to enforce religious unity. Anyone within the church who did not subscribe to Catholic Christianity was seen as a threat to the dominance and purity of the "one true faith" and they saw it as their right to defend this by all means at their disposal.
Persecution before Constantine I
In 186 BC, the Roman senate issued a decree that severely restricted the Bacchanals, ecstatic rites celebrated in honor of Dionysus. Livy records that this persecution was due to the fact that "there was nothing wicked, nothing flagitious, that had not been practiced among them" but some modern scholars suspect other reasons. Regardless, the persecution was sharp. Livy records that more people were put to death than imprisoned:
"A greater number were executed than thrown into prison; indeed, the multitude of men and women who suffered in both ways, was very considerable." 
On a bronze tablet found in Tiriolo, Italy in 1640 a Roman decree reads:
"Let none of them be minded to have a shrine of Bacchus ... Let no man, whether Roman citizen or Latin ally or other ally, be minded to go to a meeting of Bacchantes ... Let no man be a priest. Let no-one, man or woman, be a master. Let none of them be minded to keep a common fund. Let no-one be minded to make any man or woman an official or a temporary official. Henceforth let no-one be minded to conspire, collude, plot or make vows in common among themselves or to pledge loyalty to each other.
If there are any who transgress against the decrees set out above, a capital charge is to be brought against them" 
Druids were seen as essentially non-Roman: a prescript of Augustus forbade Roman citizens to practice "druidical" rites. Pliny reports that under Tiberius the druids were suppressed —along with diviners and physicians— by a decree of the Senate, and Claudius forbade their rites completely in AD 54. Druids were alleged to practice human sacrifice, a practice abhorrent to the Romans. Pliny the Elder (23 AD - 79 AD) wrote "It is beyond calculation how great is the debt owed to the Romans, who swept away the monstrous rites, in which to kill a man was the highest religious duty and for him to be eaten a passport to health."
Tiberius forbade Judaism in Rome, and Claudius expelled them from the city. However, the passage of Suetonius is ambiguous: "Because the Jews at Rome caused continuous disturbances at the instigation of Chrestus he [Claudius] expelled them from the city". Chrestus has been identified as another form of Christus; the disturbances may have been related to the arrival of the first Christians in Rome, and that the Roman authorities, failing to distinguish between the Jews and the Early Christians, simply decided to expel them all.
The Crisis under Caligula (37-41) has been proposed as the "first open break between Rome and the Jews", even though problems were already evident during the Census of Quirinius in 6 and under Sejanus (before 31). After the Jewish-Roman wars (66-135), Hadrian changed the name of Iudaea province to Syria Palaestina and Jerusalem to Aelia Capitolina in an attempt to erase the historical ties of the Jewish people to the region. In addition, after 70, Jews and Jewish Proselytes were only allowed to practice their religion if they paid the Jewish Tax, and after 135 were barred from Jerusalem except for the day of Tisha B'Av.
Though there is some debate about the responsibility for the death of Jesus, most agree that the Roman Empire played a significant part. Likewise, Christian martyrs were a significant part of Early Christianity, until the Peace of the Church in 313.
Suetonius mentions passingly that: "[during Nero's reign] Punishments were also inflicted on the Christians, a sect professing a new and mischievous religious belief" but he doesn't explain for what they were punished.
The religion of the Christians and Jews was monotheistic in contrast to the polytheism of the Romans. The Romans tended towards syncretism, seeing the same gods under different names in different places of the Empire. This being so, they were generally tolerant and accommodating towards new deities and the religious experiences of other peoples who formed part of their wider Empire. This general tolerance was not extended to religions that were hostile to the state nor any that claimed exclusive rights to religious beliefs and practice.
By its very nature the exclusive faith of the Jews and Christians set them apart from other people, but whereas the former group was in the main contained within a single national, ethnic grouping, in the Holy Land and Jewish diaspora—the non-Jewish adherents of the sect such as Proselytes and God-fearers being considered negligible—the latter was active and successful in seeking converts for the new religion and made universal claims not limited to a single geographical area. Whereas the Masoretic Text, of which the earliest surviving copy dates from the 9th century AD, teaches that "the Gods of the gentiles are nothing", the corresponding passage in the Greek Septuagint, used by the early Christian Church, asserted that "all the gods of the heathens are devils." The same gods whom the Romans believed had protected and blessed their city and its wider empire during the many centuries they had been worshipped were now demonized by the early Christian Church.
Whereas the religion of the Jews could theoretically be contained within their own nation state and pose no threat to the wider Empire, it was not so with the early Christian community which was perceived at times to be an intrinsically destabilising influence and threat to the peace of Rome, a religio illicita. The pagans who attributed the misfortunes of Rome and its wider Empire to the rise of Christianity, and who could only see a restoration by a return to the old ways, were faced by the Christian Church that had set itself apart from that faith and was unwilling to dilute what it held to be the religion of the "One True God".
After the initial conflicts between the state and the new emerging religion during which early Christians were periodically subject to intense persecution, Gallienus issued an edict of toleration for all religious creeds including Christianity, a re-affirmation of the policy of Alexander Severus.
Persecution of pagans by the Christian Roman Empire
The first episodes started late in the reign of Constantine the Great, when he ordered the pillaging and the tearing down of some pagan temples. The first anti-Pagan laws by the Christian state started with Constantine's son Constantius II, who was an unwavering opponent of paganism; he ordered the closing of all pagan temples, forbade Pagan sacrifices under pain of death, and removed the traditional Altar of Victory from the Senate. Under his reign ordinary Christians started vandalizing many of the ancient Pagan temples, tombs and monuments.
From 361 till 375, Paganism received a relative tolerance, until when three Emperors, Gratian, Valentinian II and Theodosius I, under bishop of Milan Saint Ambrose's major influence, reprised and escalated the persecution. Under Ambrose's zealous pressure, Theodosius issued the infamous 391 "Theodosian decrees," a declaration of war on paganism, the Altar of Victory was removed again by Gratian, Vestal Virgins disbanded, access to Pagan temples prohibited.
- "the traditional Roman policy, which tolerated all differences in the one loyalty" Father Philip Hughes, "History of the Church", Sheed & Ward, rev ed 1949, vol I chapter 6."
- "Two exceptions there were to the Roman State's universal toleration or indifference. No cult would be authorised which was of itself "hostile" to the State; nor any which was itself exclusive of all others, The basis of these exceptions was, once more, political policy and not any dogmatic zeal". Father Philip Hughes, "History of the Church", Sheed & Ward, rev ed 1949, vol I chapter 6.
- Religions of Rome: A History, Mary Beard, John A. North, S.R.F Price, Cambridge University Press, p234, 1998, ISBN 0-521-31682-0
- "The First Christian Theologians: An Introduction to Theology in the Early Church", Edited by Gillian Rosemary Evans, contributor Clarence Gallagher SJ, "The Imperial Ecclesiastical Lawgivers", p68, Blackwell Publishing, 2004, ISBN 0-631-23187-0
- Livy, http://www.fordham.edu/halsall/ancient/livy39.html
- Decree of the Senate Concerning the Rites of Bacchus ,http://www.forumromanum.org/literature/sc/sc_bacch_e.html
- Pliny's Natural History xxx.4.
- Suetonius, The Twelve Caesars, Life of Claudius paragraph 25
- "The Britons", Christopher Allen Snyder, p52, Blackwell Publishing, 2003, ISBN 0-631-22260-X
- Suetonius, The Twelve Caesars, Life of Tiberius paragraph 36
- H.H. Ben-Sasson, A History of the Jewish People, Harvard University Press, 1976, ISBN 0-674-39731-2, The Crisis Under Gaius Caligula, pages 254–256: "The reign of Gaius Caligula (37-41) witnessed the first open break between the Jews and the Julio-Claudian empire. Until then — if one accepts Sejanus' heyday and the trouble caused by the census after Archelaus' banishment — there was usually an atmosphere of understanding between the Jews and the empire ... These relations deteriorated seriously during Caligula's reign, and, though after his death the peace was outwardly re-established, considerable bitterness remained on both sides. ... Caligula ordered that a golden statue of himself be set up in the Temple in Jerusalem. ... Only Caligula's death, at the hands of Roman conspirators (41), prevented the outbreak of a Jewish-Roman war that might well have spread to the entire East."
- H.H. Ben-Sasson, A History of the Jewish People, Harvard University Press, 1976, ISBN 0-674-39731-2, page 334: "In an effort to wipe out all memory of the bond between the Jews and the land, Hadrian changed the name of the province from Iudaea to Syria-Palestina, a name that became common in non-Jewish literature."
- Suetonius, The Twelve Caesars, Life of Nero paragraph 16
- Tacitus, Annals XV.44
- In the earliest extant manuscript containing Annales 15:44, the second Medicean, the e in "Chrestianos", Chrestians, has been changed into an i; cf. Gerd Theißen, Annette Merz, Der historische Jesus: ein Lehrbuch, 2001, p. 89. The reading Christianos, Christians, is therefor doubtful.
- "the word 'Hellenism' was used by the Christian elites in the Greek East alongside the universal derogatory terms 'polytheism' and 'idolatry' to describe Graeco-Roman religion" see "Pagan Monotheism in Late Antiquity, Polymnia Athanassiadi, Michael Frede, Contributor Polymnia Athanassiadi, p7, Oxford University Press, 2002, ISBN 978-0-19-815252-1
- "A History of the Church", Philip Hughes, Sheed & Ward, rev ed 1949, vol I chapter 6.
- "The Greek Septuagint translated into English", psalm 95:5, translated by Sir Lancelot Charles Lee Brenton, 1851. Jerome would follow the Greek text rather than the Hebrew when he translated the Latin Vulgate edition of the bible. The "devils" epithet would still appear in bibles until the end of the 20th century when the consensus reverted back to the original Hebrew text for modern translations
- A modern Christian writes that the gods of the pagans are "in fact fallen angels (otherwise known as devils) ... And that is what the pagans, then as now, serve as "gods" ", Roy H. Schoeman, "Salvation is from the Jews", Ignatius Press, 2003, ISBN 0-89870-975-X
- "Devil Worship", Catholic Encyclopedia, 1908 Edition
- The modern Church takes a much less antagonistic stance to non-Abrahamic faiths. see Dignitatis humanae and Nostra aetate
- "Julian the Apostate and His Plan to Rebuild the Jerusalem Temple", Jeffrey Brodd, Biblical Archaeology Society, Bible Review, October 1995.
- "The Memorial of Symmachus"
- "Letter of Ambrose to the Emperor Valentinian", The Letters of Ambrose Bishop of Milan, 384AD, retrieved 5 May 2007.
- R. MacMullen, "Christianizing The Roman Empire A.D.100-400, Yale University Press, 1984, ISBN 0-300-03642-6
- Eusebius Pamphilius and Schaff, Philip (Editor) and McGiffert, Rev. Arthur Cushman, Ph.D. (Translator) NPNF2-01. Eusebius Pamphilius: Church History, Life of Constantine, Oration in Praise of Constantine quote: "he razed to their foundations those of them which had been the chief objects of superstitious reverence"
- Kirsch, J. (2004) God against the Gods, pp. 200–1, Viking Compass
- "The Codex Theodosianus On Religion", XVI.x.4, 4 CE
- Sheridan, J.J. (1966) The Altar of Victor – Paganism's Last Battle. in L'Antiquite Classique 35 : 186-187.
- Ammianus Marcellinus Res Gestae 22.4.3
- Sozomen Ecclesiastical History 3.18.
- Theodosian Code 16.10.3
- Theodosian Code 9.17.2
- Byfield (2003) pp. 92–4 quote:
In the west, such [anti-Pagan] tendencies were less pronounced, although they had one especially powerful advocate. No one was more determined to destroy paganism than Ambrose, bishop of Milan, a major influence upon both Gratian and Valentinian II. ... p.94 The man who ruled the ruler - Whether Ambrose, the senator-bureaucrat-turned-bishop, was Theodosius's mentor or his autocrat, the emperor heeded him--as did most of the fourth-century church.
- MacMullen (1984) p.100 quote:
The law of June 391, issued by Theodosius ... was issued from Milan and represented the will of its bishop, Ambrose; for Theodosius--recently excommunicated by Ambrose, penitent, and very much under his influence43--was no natural zealot. Ambrose, on the other hand, was very much a Christian. His restless and imperious ambition for the church's growth, come what might for the non-Christians, is suggested by his preaching.
- King (1961) p.78
- H.A.Drake, Lambs into Lions: explaining early Christian intolerance, Past and Present 153 (1996), 3-36, Oxford Journals
- Peter Garnsey, Religious Toleration in Classical Antiquity, in: W.J.Sheils (Ed.), Persecution and Toleration, Studies in Church History 21 (1984), 1-27
- Ramsay MacMullen, Christianizing the Roman Empire: AD 100-400 (1989)
- ——, Christianity and Paganism in the Fourth to Eighth Centuries (1997) ISBN 0-300-08077-8