Chapter 11 [VII.]— They Who Have Not Received the Gift of Perseverance, and Have Relapsed into Mortal Sin and Have Died Therein, Must Righteously Be Condemned.
If, then, these things be so, we still rebuke those, and reasonably rebuke them, who, although they were living well, have not persevered therein; because they have of their own will been changed from a good to an evil life, and on that account are worthy of rebuke; and if rebuke should be of no avail to them, and they should persevere in their ruined life until death, they are also worthy of divine condemnation for ever. Neither shall they excuse themselves, saying—as now they say, “Wherefore are we rebuked?”— so then, “Wherefore are we condemned, since indeed, that we might return from good to evil, we did not receive that perseverance by which we should abide in good?”
They shall by no means deliver themselves by this excuse from righteous condemnation. For if, according to the word of truth, no one is delivered from the condemnation which was incurred through Adam except through the faith of Jesus Christ, and yet from this condemnation they shall not deliver themselves who shall be able to say that they have not heard the gospel of Christ, on the ground that “faith comes by hearing,” how much less shall they deliver themselves who shall say, “We have not received perseverance!”
For the excuse of those who say, “We have not received hearing,” seems more equitable than that of those who say, “We have not received perseverance;” since it may be said, O man, in that which you had heard and kept, in that you might persevere if you would, but in no wise can it be said, That which you had not heard you might believe if you would.
Source: On Rebuke and Grace (New Advent)