Nicene and Post-Nicene Fathers, Vol. III:
Moral Treatises of St. Augustin: Section 24

24. 2699 But it may well be asked, whether this patience likewise be the gift of God, or to be attributed to strength of the human will, by which patience, one who is separated from the Church doth, not for the error which separated him but for the truth of the Sacrament or Word which hath remained with him, for fear of pains eternal suffer pains temporal. For we must take heed lest haply, if we affirm that patience to be the gift of God, they in whom it is should be thought to belong also to the kingdom of God; but if we deny it to be the gift of God, we should be compelled to allow that without aid and gift of God there can be in the will of man somewhat of good. Because it is not to be denied that it is a good thing that a man believe he shall undergo pain of eternal punishment if he shall deny Christ, and for that faith endure and make light of any manner of punishment of man’s inflicting.

Footnotes

See on Profit of Believing, c. 1, p. 347.