Disciple I see clearly several meanings of this word heretic, but in some of them it is seldom used, though heretics are often mentioned. Say, therefore, if you have heard or seen a description of this word heretic in the meaning in which it is more commonly used.

Master Perhaps you mean to speak of the heretic who is excommunicate, and, if he has been lawfully convicted and has not corrected himself according to the Church's form, should be handed over to a secular court.

Disciple Describe him, if you can.

Master Some describe [him] saying this: A heretic is someone seriously baptized, or behaving as someone baptized, who pertinaciously doubts or errs against Catholic truth.

Disciple Clarify the elements [of this description], and explain which persons they intend to include and which to exclude.

Master By the first element, when they say "seriously baptized", they exclude those baptized in jest who are considered as not baptized. And they include by the same element not only Christians baptized by Catholics, but also those baptized by heretics in the Church's form -- and outside the Church's form (who in no way receive the sacrament of baptism in respect of either grace or character). By the same element they exclude Jews, Saracens and pagans who have never been baptized and have not behaved as baptized persons, who should not at all be punished by the penalty of the heretics of whom you were speaking.

And by the second element, when it is said "behaving as someone baptized", they include those who think or pretend that they have been baptized and live among Christians as if baptized. If these withdraw from the faith they should be punished as if they had truly been baptized.

By the third element, when it is said, "who errs or doubts pertinaciously against Catholic truth", are excluded all those who either doubt or err against the faith from simplicity or ignorance alone, without any pertinacity. For such persons should not be considered heretics, but should be diligently informed about the faith, and if afterwards they doubt or err pertinaciously they should be condemned as heretics.

William of Ockham, Dialogus
part 1, book 3, chapters 1-5.

Text by John Kilcullen and John Scott,
Translation by John Kilcullen.