Luther once said that the church never burned heretics (meaning the true church). Cardinal Bellarmine in debate at a later date referred to Luther’s statement, he thought he was referring to the Catholic Church and said, “This argument proves not the sentiment, but the ignorance or impudence of Luther; for as almost an infinite number were either burned or otherwise put to death, Luther either did not know it, and was therefore ignorant; or if he knew it, he is convicted of impudence and falsehood-for that heretics were often burned by the church, may be proved by adducing a few from many examples.” John Dowling, The History of Romanism, p. 547
Pope Martin V in a letter to the King of Poland said, “burn, massacre, make deserts everywhere, for nothing could be more agreeable to God, or more useful to the cause of kings, than the extermination of the Hussites” L M de Cormenin, The Public and Private History of the Popes of Rome, Vol. II, pp. 116,117
The Catholic Encyclopedia acknowledges that the inquisition, especially in Spain towards the close of the Middle Ages was “one of the darker chapters in the history of the church”. It admits to about 5-6000 deaths of protestants. (New Catholic Encyclopedia, “Inquisition”, “Auto-da-Fe”, “St. Bartholomew’s Day, Massacre of”)
However, these figures are very modest, overlooking the crusades against the Albigenses and Waldenses, and omitting such things as the Thirty Year War, in which military and civilian casualties, Protestant and Catholic exceeded 8 million.
There is no doubt that terrible things happened in the past, which is why it is so important to uphold the principle of religious liberty, and let people practice their religion whether Catholic or Protestant according to their conscience.