‘Dominant in the Middle Ages was the view that while non-Christians could be tolerated (for otherwise any form of coexistence with Islam would have been impossible), Christians, who had fallen into heresy, could not. Thomas of Aquino maintained that heretics deserved to be executed, for were not counterfeiters sentenced to death. Distortion of the faith that gave life to the soul was, indeed, a much fouler crime than forging money. Pagans did not see the light, whereas heretics had turned away from it and had thereby committed a crime against the Holy Ghost and their own conscience. Thomas of Aquino added that heretics should be forced, even by physical coercion, to do what they had promised and keep what they had once acknowledged. And this view Catholicism has maintained down the centuries. This was the reason why Pope Gregory XVI had declared freedom of conscience a madness in 1832, and why in 1864 it was officially condemned by Rome (during the reign of Pope Pius IX).’
Yefim Chernyak: Ambient Conflicts, Progress, (1978), Page 58.