Heresy
Heresy is the obstinate denial by a professed, baptized Christian of a revealed truth or of that which the Roman Catholic Church has proposed as a revealed truth. The unbaptized person is incapable of heresy, and the baptized person is not guilty of “formal” but only of “material” heresy if he does not know that he denies a revealed truth. The seriousness with which Roman Catholicism regarded heresy is shown by the ancient penalty of excommunication. Civil penalties, including death, did not appear until the age of Constantine. Lesser civil disabilities continued in force, though the law was often ignored, into the 20th century. Protestant governments were often as severe as Roman Catholic governments in the suppression of heresy.
Roman Catholic theologians often deal with heresy, paradoxically, as a necessary step in the development of dogma. They point out that the questions raised by heresy are often legitimate, though heretics too quickly assume a one-sided and exclusive view of the doctrine they wish to impose on the entire church. Modern studies have noted that many of the criticisms of the church made by the heretics of the early 11th century were made by the papal reformers after 1050. In recent times many of the theses of Modernism, which were condemned vigorously by Pius X in 1907, found their way into Catholic theology later in the 20th century.