It's in his 2nd letter
II Thessalonians 2:1 "Now we beseech you, brethren, by the coming of our Lord Jesus Christ, and by our gathering together unto him,"
Paul is now getting to the purpose for this second letter. Paul reminded them of his first letter to them, of Christ returning and our gathering back to Him. Because of the misunderstanding that could be drawn from the first letter, he writes the following.
II Thessalonians 2:3 "Let no man deceive you by any means: for that day shall not come [it will not be], except there come a falling away first, and that man of sin be revealed, the son of perdition;"
Jesus Christ is not gathering anyone to Him, nor is He coming here to earth, except there be a falling away first. The word for "falling away", in the Greek is "apostasy". Jesus told us in His revelation to John, in Revelation 9:11, that "Apollyon" is one of Satan's names, and this name is derived from the word apostasy.
The word "perdition" means "one that perishes". Satan and only Satan has already been condemned to die [to perish] by God. Satan's judgment day has come already back in the first earth age, and he will have no part in the Great White Throne judgment, or any other judgment yet to come. He has already been judged, and sentence by God to death "perdition" [Ezekiel 28 covers this judgment].
What did Paul say again?
Jesus Christ is coming back to this earth, but the saints of Christ will not be gathered to Him until the "son of perdition" [Satan] be revealed first.