he who opposes and exalts himself against all that is called God or that is worshiped; so that he sits as God in the temple of God, setting himself up as God.
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).