Let no one deceive you in any way. For it will not be, unless the departure comes first, and the man of sin is revealed, the son of destruction,
It seems that Paul referred here to the departure from the Christian faith of professing (not genuine) Christians soon after the Rapture, at the beginning of the day of the Lord. This was not the same apostasy he and other apostles spoke of elsewhere when they warned of departure from the faith before the Rapture (1 Timothy 4:1-3; 2 Timothy 4:3-4; James 5:1-8; 2 Peter 2; 2 Peter 3:3-6; Jude).