Before around 1500 A.D., no one had heard of OSAS. John Calvin first proposed it. And he said he got his idea of "perseverance of the saints" (the "P" in TULIP) from St. Augustine. He probably did, but he probably misunderstood it.
"
Chapter 9.— When Perseverance is Granted to a Person, He Cannot But Persevere.
Now, moreover, when the
saints say, Lead us not into
temptation, but deliver us from
evil,
Matthew 6:13 what do they
pray for but that they may persevere in
holiness? For, assuredly, when that gift of God is granted to them — which is sufficiently plainly shown to be God's gift, since it is asked of Him — that gift of
God, then, being granted to them that they may not be led into
temptation, none of the
saints fails to keep his perseverance in
holiness even to the end. For there is not any one who ceases to persevere in the
Christian purpose unless he is first of all led into
temptation. If, therefore, it be granted to him according to his
prayer that he may not be led, certainly by the gift of God he persists in that sanctification which by the gift of God he has received." From:
https://www.newadvent.org/fathers/15122.htm
Augustine is not saying what Calvin thought: not that, once a man is justified, he can never lose grace, otherwise he was never justified in the first place.
Rather, Augustine is saying, after justification, one must pray daily, e.g. the Lord's Prayer, for the Gift of Perseverance. Then, at some time later, if that Gift of Perseverance is given to them by God, they will in fact persevere to the end in holiness and not be lost. That's clear.
Salvation=Justification+Perseverance, not Justification alone. Those who treat justification alone as salvation find difficulty in explaining the OP question.
And so neither of the two options that (1) even though a man becomes an apostate/atheist, he is still saved, or (2) if a man becomes a man an atheist, he was never saved to begin with, is right imo.
Rather, all that can be legitimately concluded/inferred is (1) if someone became an atheist, he may indeed have been truly justified in the beginning; but (2) he never sufficiently prayed, and thus did not receive the Grace of Perseverance in Faith.
That's why in Hebrews 10, and other verses, there are so many exhortations to Persevere, in order to obtain the Promise.
God Bless.
