2_Thessalonians_2v3
The Rebellion aka The Falling Away
2 Thessalonians 2:3
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.
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.