except there come a falling away first and that man of sin be revealed

Seeing it is a righteous thing with God to recompense tribulation to them that trouble you;

And to you who are troubled
    rest with us,

when the Lord Jesus shall be revealed from heaven
      with his mighty angels,
      in flaming fire
      taking vengeance on them that know not God, and that obey not the gospel of our Lord Jesus Christ: Who shall be punished with everlasting destruction from the presence of the Lord, and from the glory of his power;

When he shall come to be glorified in his saints,
     and to be admired in all them that believe (because our testimony among you was believed) in that day.

Wherefore also we pray always for you, that our God would count you worthy of this calling, and fulfil all the good pleasure of his goodness, and the work of faith with power:

That the name of our Lord Jesus Christ may be glorified in you, and ye in him, according to the grace of our God and the Lord Jesus Christ.

Now we beseech you, brethren, by the coming of our Lord Jesus Christ, and by our gathering together unto him,

That ye be not soon shaken in mind, or be troubled,
     neither by spirit,
     nor by word,
     nor by letter as from us,
as that the day of Christ is at hand.

Let no man deceive you by any means: for that day shall not come, 

[What day?  The day of the coming of our Lord Jesus Christ, and by our gathering together unto Him!]

except 
1) there come a falling away first, 
2) and that man of sin be revealed, the son of perdition; Who opposeth and exalteth himself above all that is called God, or that is worshipped; so that he as God sitteth in the temple of God, shewing himself that he is God.

Remember ye not, that, when I was yet with you, I told you these things?

And now ye know what withholdeth that he might be revealed in his time.

For the mystery of iniquity doth already work: only he who now letteth will let, until he be taken out of the way.

And then shall that Wicked be revealed, whom the Lord shall consume with the spirit of his mouth, and shall destroy with the brightness of his coming:

Even him, whose coming is after the working of Satan with all power and signs and lying wonders,

And with all deceivableness of unrighteousness in them that perish; because they received not the love of the truth, that they might be saved.

And for this cause God shall send them strong delusion, that they should believe a lie:

That they all might be damned who believed not the truth, but had pleasure in unrighteousness.

2 Thessalonians 1:6-12
2 Thessalonians 2:1-12