The battle for doctrinal control of Christianity began with the declarations of St. Paul in the New Testament. In the religion's first three centuries, numerous sects, many arising from Gnosticism, were in conflict. The first Council of Nicaea (A.D. 325), which addressed the challenge of Arianism, was among convocations at which a Christian orthodoxy was established.
Excommunication was the usual method of dealing with heretical individuals or small groups. The medieval church undertook military action (as against the Albigenses, in 1208) and extensive legal and punitive campaigns (such as the Inquisition) in striving to suppress large-scale heresy. The Protestant Reformation created new churches that at first campaigned against heresy from their own doctrinal bases; over time, however, the Roman Catholic church has remained the only Christian body that has continued with any frequency, on the basis of canon law, to prosecute heretics.
See also blasphemy.
The Americanist heresy is defined as the endorsement of what were thought to be anti-Catholic principles embraced by the United States: absolute freedom of the press, liberalism, individualism, complete separation of church and state, etc. These were condemned by the popes of the time. The most notable and concise condemnation was in the Syllabus of Errors of Pope Pius IX.
The Great Irish Famine (1845-1849) caused a mass exodus of Irish Catholics to the United States, causing Catholicism to become the United States' largest single form of Christianity. America remains majority-Protestant, and that was more true then than now. Discrimination against the Irish immigrants led them to seek assimilation into U.S. culture.
The Irish Americans formed the majority of Catholics, and therefore had the most bishops. The bishops largely shared the view that freedom of religion is a nobler idea than simple religious tolerance. In America they were much freer than in British-run Ireland which had an established Church. So the separation of Church and State seemed like a good idea for many Irish-Americans.
This issue was brought forcefully to the attention of European Catholics by Comptesse de Ravilliax's translation of a biography of Isaac Thomas Hecker by Walter Elliott, with the introduction by Abbe Felix Klein drawing the most ire from the Vatican. Father Hecker had been dead for years at this point and had never been viewed by the Pope with disfavor. However this translation of Hecker's work and introductions to this book and to the book about him made him appear to have been much more of a radical than he in fact was.
Hence Pope Leo XIII intervened with the encyclical Testem Benevolentiae Nostrae addressed to James Cardinal Gibbons, Archbishop of Baltimore. Two years before he did so, though, Pope Leo XIII indicated a generally positive view of the USA in the encyclical Longinqua. This work commented most on the success of Catholicism in the US but noted the view that the Church "would bring forth more abundant fruits if, in addition to liberty, she enjoyed the favor of the laws and the patronage of the public authority." Catholicism had long allowed nations to tolerate other religions, but Popes traditionally felt Catholicism must be favored as the true faith when possible. More relevant to this controversy, Pope Leo XIII expressed concerns about the liberalism of American Catholics: he pointed out that the faithful could not decide doctrine for themselves. He also emphasized that Catholics should obey the teaching authority of the Church. In general, he deemed exposing children to public schools as something to be avoided when possible. In fairness, public schools at this time still required the saying of Protestant prayers and reading of the King James Bible. The Pope derided the idea that all opinions should be aired publicly, as he felt certain speech could harm general morality. He condemned the biography of Hecker and Americanism.