38 And in his next letter: "Meanwhile I keep crying out, 'Any man who is joined to Peter's Chair, he is my man'." Since he had learnt this "rule of faith" from his study of the Bible, he was able to refute a false interpretation of a Biblical text with the simple remark: "Yes, but the Church of God does not admit that." When, again, Vigilantius quoted an Apocryphal book, Jerome was content to reply: "A book I have never so much as read! For what is the good of soiling one's hands with a book the Church does not receive?" With his strong insistence on adhering to the integrity of the faith, it is not to be wondered at that he attacked vehemently those who left the Church; he promptly regarded them as his own personal enemies. "To put it briefly," he says, "I have never spared heretics, and have always striven to regard the Church's enemies as my own." To Rufinus he writes: "There is one point in which I cannot agree with you: you ask me to spare heretics - or, in other words - not to prove myself a Catholic." Yet at the same time Jerome deplored the lamentable state of heretics, and adjured them to return to their sorrowing Mother, the one source of salvation; he prayed, too, with all earnestness for the conversion of those "who had quitted the Church and put away the Holy Spirit's teaching to follow their own notions."
Source: Spiritus Paraclitus (Vatican.va)