… that Day will not come unless first comes the falling away, and the man of sin is revealed, the son of perdition, (2 The 2:3)
In 2 Thes, Paul is writing what must happen before Jesus comes:
2:1 Now we ask you, brothers, regarding the coming of our Lord Jesus Christ and our gathering together to him
for you not to be quickly shaken in the mind, nor to be disturbed, neither through a spirit, nor through speech, nor through letter, as through us, as if the Day of Christ has come.
3, Do not let anyone deceive you in any way, for it will not come unless the apostasy comes first, and the man of lawlessness is revealed, the son of destruction,
4 he opposes and exalts himself above every so-called god or object of worship. As a result, he [the antichrist] seats himself in the temple of God and declares himself to be God.
There is, therefore, to take place a great apostasy in Christendom. Before the arrival of the Lord’s Day, the Antichrist (man of sin, son of perdition), who declares himself to be God, will appear and reside in the temple of God. Jesus will not come before these happen. It seems as if apostasy has been actualized widely, but the Antichrist has not yet appeared. Settling as being proclaimed God (in the new Temple to be built in Jerusalem) is not yet in sight.