Summa contra Gentiles3.157 Chapter CLVII. That man cannot be freed from sin save by grace

From the same premisses it can be shown that man cannot arise from mortal sin except by grace.

Because by mortal sin man turns away from his last end: and by grace alone is man directed towards his last end. Therefore by grace alone can he arise from sin.

Again. Offence is not removed save by love. Now, by mortal sin man offends God: for it is written that God hateth sinners, inasmuch as it is His will to deprive them of the last end, which He prepares for those whom He loves. Therefore man cannot arise from sin except by grace, which causes a kind of friendship between God and man.

The same conclusion follows from all the arguments given above to prove the necessity of grace.

Hence it is said (Isa. xliii. 25): I, even I, am He that blot out thy iniquities for My own sake: and (Ps. lxxxiv. 3): Thou hast forgiven the iniquity of Thy people; Thou hast covered all their sins.

Hereby is refuted the error of the Pelagians who said that man can arise from sin by his free-will.