Section — Noldin, Summa of Moral Theology
Section 1. Defection from Faith.
491. Because of Defection from Faith these are irregular: apostates from the faith, heretics, and schismatics.1
a. For making a just judgment about this irregularity, attention must be paid to the notions as they are determined in the Code: if someone after receiving baptism: alpha. totally recedes from the Christian faith, he is an apostate; beta. while retaining the Christian name, pertinaciously denies or doubts some truth to be believed by divine and Catholic faith, he is a heretic; gamma. if he refuses to be subject to the supreme pontiff or refuses to communicate with members of the Church subject to him, he is a schismatic.2
b. From these definitions it is gathered that to contract this irregularity it is not necessary that someone give his name to an acatholic sect, provided that he denies the Catholic faith either wholly or in part, even if he does not outwardly break communion with the Catholic Church.
A Catholic Christian who denies or calls into doubt the infallibility of the Roman pontiff is irregular, provided he outwardly manifests heresy.
c. He who lives in a heretical or schismatic sect in good faith, whether he is a material heretic or schismatic only, does not contract irregularity, because his delict is not a grave sin.
1 Cn. 985, 1.
2 Cn. 1325, 2.