Eschatology
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, so that he takes his seat in the temple of God, proclaiming himself to be God. - 2 Thessalonians 2:3-4
Paul wrote 2 Thessalonians to assure the church that the day of the Lord had not yet come. The rebellion and the reveal of the man of lawlessness will occur before the day of the Lord.
(2 Thessalonians 2:3)
Etymology
ἀποστασία apostasia