Catholic Answers Senior Apologist Jimmy Akin describes the circumstances behind a saint who was also an anti-pope.
Jimmy Akin was born in Texas and grew up nominally Protestant. At age 20 he experienced a profound conversion to Christ. Planning on becoming a Protestant pastor or seminary professor, Jimmy started an intensive study of the Bible, but the more he immersed himself in Scripture, the more he found...
"All heretics wish to be styled Catholic, yet if anyone asks them where is the Catholic place of worship none would venture to point out his own."
~ Augustine of Hippo, convert, bishop, theologian, Doctor of the Church; using the early Christian consensus on the name of the Catholic Church to show how even heretics know the true church (from the article Catholic)