2Th 2:3-4
"Let no one deceive you in any way. For that day will not come, unless the rebellion comes first, and the man of lawlessness is revealed, the son of destruction, who opposes and exalts himself against every so-called god or object of worship."
Two events precede the Day of the Lord: (1) the rebellion (apostasia); (2) the revelation of the 'man of lawlessness' (anthropos tes anomias). This figure enthrones himself in God's temple, declaring himself to be God — the height of blasphemy. Historical identifications: Caligula, Nero, the eschatological Antichrist.
2Th 2:7-8
"For the mystery of lawlessness is already at work. Only he who now restrains it will do so until he is out of the way. And then the lawless one will be revealed, whom the Lord Jesus will kill with the breath of his mouth."
The 'mystery of lawlessness' (mysterion tes anomias) is already at work. The one who restrains (katechon) — identity debated: the Holy Spirit, the Roman Empire, the Church. Christ will destroy the lawless one 'with the breath of his mouth' — God's word is sufficient to destroy any power of evil.