Heresy is either material or formal. The material heretic denies the authority of the Church to declare the faith, but without pertinacity… The formal heretic has a sufficient knowledge of the Church’s authority, but refuses to accept the faith of the true Church, and denies it in at least one point.

These are the signs that are customarily given by which one may judge someone to be a material heretic:-

1. If he would be ready to submit himself to the Church’s judgement when he knew his error, even if in the meantime he tenaciously defends his own opinion.

2. If he knows nothing about the Catholic faith, and has never wondered about it.

3. If, wondering about it, he has sought to know the truth as far as he was able.

Now, if someone delays his conversion from human respect or from carelessness, he is not on that account a heretic; yet he sins against the positive commandment of acquiring the faith, if he delays for a long time.

(Benedict Merkelbach, Summa Theologiae Moralis, I, 746)