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
AKA the lawless one, the son of destruction.
The mystery of lawlessness is already at work, but the church of the Thessalonians knew he who was restraining the lawless one so that he may be revealed in his time. Once he who restrains him is out of the way and the lawless one is revealed, his coming will be by the activity of Satan with all power and false signs and wonders. God will send the perishing a strong delusion, so they may believe what is false and fall for the lawless one's wicked deception.
He will oppose and exalt himself against every so-called god or object of worship. He will take his seat in the temple of God, proclaiming himself to be God.
The Lord Jesus will kill him with the breath of his mouth and bring to nothing by the appearance of his coming.
(2 Thessalonians 2:1-12)
The day of the Lord
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. Also, since the lawless one will take his seat in the temple of God, there must also be a rebuilding of the temple, no?
(2 Thessalonians 2:3-4)
Who restrains the lawless one?
As Paul describes what will take place before the day of the Lord, he says, "Do you not remember that when I was still with you I told you these things? And you know what is restraining him now so that he may be revealed in his time." He probably explained all of this and more when he was with them, including who restrains the lawless one.
(2 Thessalonians 2:5-6)
I think it's the Holy Spirit.
Who will fall for the lawless one?
The lawless one's wicked deception is for those who are perishing, because they refused to love the truth and so be saved. God will send them a strong delusion, so that they may believe what is false, in order that all may be condemned who did not believe the truth but had pleasure in unrighteousness.
(2 Thessalonians 2:10-12)