St Thomas, characteristically, proves his thesis by four a priori arguments, and one of testimonies from Scripture. We may rest satisfied with the one too common argument of experience. He continues: "Hereby is excluded the error of certain heretics (St Jerome, Contra Jovin. Chap. XXXI, says it was an error of Jovinian), that man, after receiving the grace of the Holy Ghost, cannot sin; and that if he sins, he never had the grace of the Holy Ghost." The only possibility of heresy in such a plain matter is by taking sanctifying grace to consist in nothing else than election and predestination to glory (cf. B. III, Chap. CLXIV). In the tautological sense that all whom God foresees in the ranks of the Blessed will be finally Blessed, it is true to say that 'once elect, always elect.' But it would be heresy to say that sin is never imputed to the elect. Oliver Cromwell, dying, asked a minister, whether it was possible to fall from grace. The Calvinist minister at his bedside said that it was not possible. "Then," said the dying man, "I am safe, for I was in grace once." How did he know that? Was it provable from a baptismal register? Not to a Calvinist.
St Thomas concludes the chapter thus: "The text 1 John iii, 6, 9, means that any one who is adopted as a son, or born again as a son of God, receives gifts of the Holy Ghost, which of themselves are powerful enough to keep the man from sin, nor can he sin so long as he lives according to them: he may however act contrary to them, and by sinning depart from them. He that is born of God, cannot sin, in the same way that 'warm water cannot chill one,' or 'the just man never acts unjustly,' to wit, in so far as he is just." In other words, the text holds good in sensu composito, not in sensu diviso.
Of God and His Creatures: 4.70