[Heretics] deemed a sin committed by a man after he had received pardon, to be so grave that it could not be forgiven. In this they erred not only with regard to sin which, even after a sin has been forgiven, can be either more or less grievous than the first, which was forgiven, but much more did they err against the infinity of Divine mercy, which surpasses any number and magnitude of sins, according to Psalm 50:1-2: "Have mercy on me, O God, according to Thy great mercy: and according to the multitude of Thy tender mercies, blot out my iniquity."
— Saint Thomas Aquinas, Summa Theologica, Pars. III Q. 84 Art. 10