The falling away or apostasy will coincide with the man of sin, whom Paul also calls the son of perdition (2 Thess 2:3) and the lawless one (2 Thess 2:8). Paul writes that this man’s revelation (apokalupthe) suggests he will appear to be something different prior to his apostasy. It’s not his birth, but his revealing which is important and will begin the Great Tribulation (Mt 24; Rev 5-10).

I. His Character (2 Thess 2:3). This man is a human, not a personification. John called him the antichrist (1 Jn 2:18, 22; 4:3; 2 Jn 7). The word anti has two aspects. First, anti means against; the antichrist is against Christ; second, anti means substitute; the antichrist is a substitute Christ. This end-time character will be against Jesus and offer himself as a replacement Jesus.
Paul described this man by two fascinating titles. The title son was a common Jewish figure of speech to describe the primary characteristic of a person, something that was shared from a father. First, he’s identified as the man of sin (anomia) describes the man’s character. Sin is his “father”. He will be like sin in human flesh without any light of God’s image. He’s also characterized by perdition (apoleia) meaning ruin, waste, or lostness. Perdition is used in the New Testament of things or people eternally damned (Mt 7:13; Acts 8:20; Rom 9:22; Phil 3:19; 2 Pet 2:1; Rev 17:8, 11), including Judas Iscariot (Jn 17:12).

II. His Type. Judas Iscariot was a type or illustration of the man of sin. Judas knew Jesus personally, experienced Him as only eleven others did, yet rejected Jesus, His miracles, His teaching, and everything about Him. Judas was an apostle (Lk 6:13, 16), preached the gospel and performed miracles (Mk 3:13-19); none of these are evidences of salvation (2 Cor 11:13-15; 2 Thess 2:9-10; Rev 13:11-18; 16:14). Judas had the other 11 apostles fooled to his identity, but Jesus knew him from the beginning (Jn 6:64). For all the greatness of his ministry, Judas betrayed the Lord (Jn 13:21-26), was a thief (Jn 12:6), a deceiver (Mt 26:14-16), and demon possessed (Jn 13:27). Upon his suicide, Judas went to his own place (Acts 1:16-20, 25) and was better off never being born (Mt 26:34).
Rather than the gospel bringing Judas to faith, the gospel hardened him. As horrible as Judas’ apostasy was, the man of sins’ will be worse.

III. His Actions (2 Thess 2:4-5). The son of perdition will display his apostasy by opposing (antikeimenos, an open hostility) and exalting (huperairomenos, to place one at the highest point possible over another) himself above God and what is worshiped. He will do this in the temple of God, sit as God, showing himself as if he is God. He will not only be against God, but will be a substitute god. He will be, by very definition, the antichrist.
The Church has never had a temple building; we are the temple (1 Cor 3:16-17; 6:19-20). In the Bible, the temple of God is always associated with Judaism.
Paul adds he’d taught these things while present in Thessalonica; it wasn’t new to the church. Eschatology, the study of end time events, was a significant issue of doctrine for Paul if he taught so much of it personally.
As believers in Jesus as our Saviour and coming King, we have no need to fear the apostasy of the man of sin of the last days. We will not be overtaken and defeated by these things (1 Thess 5:2, 4). The church is waiting for the return and our gathering together unto Jesus Christ not the revelation of the antichrist (1 Thess 4:15-18).