That Day will not come unless the FALLING AWAY comes first

[tonychanyt]

Paul visited James and the brothers in Jerusalem in Acts 21. They updated him of the accusation against him, Berean Literal Bible:

21 ""Now they have been informed about you, that you teach all Jews among the Gentiles apostasy from Moses, telling them not to circumcise the children nor to walk in the customs."

They accused Paul of teaching rebellion against Judaism/Moses.

Strong's Greek: 646. ἀποστασία (apostasia) — 2 Occurrences

BDAG:

defiance of established system or authority, rebellion, abandonment, breach of faith

2 Thessalonians 2:

3 No one should deceive you in any way, because it is not until the apostasy shall have come first, and the man of lawlessness shall have been revealed—the son of destruction, 4 the one opposing and exalting himself above every so-called god or object of worship—so as for him to sit down in the temple of God, setting forth that he himself is God.

There will be a future rebellion/apostasy against Christ.

New King James Version:

Let no one deceive you by any means; for that Day will not come unless the falling away comes first, and the man of sin is revealed, the son of perdition,

More accurately, the Greek refers to the apostasy/rebellion.