OSAS
Scriptures Which Refute 'Once Saved Always Saved'
Jesus
Only Those Who Persevere To The End Will Be Saved
But the one who perseveres to the end will be saved.
This directly opposes those who teach that once we are saved, we are always saved. Jesus said only those who endure to the end shall be saved.
Paul
1_Corinthians_10v12
Be Careful Not To Fall
So the one who thinks he is standing firm should be careful not to fall.
Paul never would have needed to give this warning if it were not possible for a believer to fall.
The Rebellion aka The Falling Away
Let no one deceive you in any way, for it will not come until the rebellion occurs and the man of lawlessness -- the son of destruction -- is revealed.
Strong's Concordance
Definition - Apostasy, rebellion, defection, falling away
Word Origin - Derived from ἀφίστημι (aphistēmi), meaning "to lead away, to depart, to fall away.
Usage - The term ἀποστασία is used in the New Testament to describe a departure from the faith or a rebellion against established religious beliefs. It is often associated with a significant turning away from God and His teachings.
Helps Word-studies
646 apostasía (from 868 /aphístēmi, "leave, depart," which is derived from 575 /apó, "away from" and 2476 /histémi, "stand")–properly, departure (implying desertion); apostasy –literally, "a leaving, from a previous standing."
The Greek word for "rebellion" (or "falling away" in some translations) is apostasia which literally means "a leaving, from a previous standing."
One had to have been a believer at one time in order to later turn away from God and His teachings.
The false teaching that those who fall away were never really Christians just doesn't stand.
If they had never been a Christian, then there is nothing to fall away from.
Only a one-time believer can fall away from the faith.