2 Thessalonians 2: 2-12 “That you not be quickly shaken from your composure or be disturbed either by a spirit or a message or a letter as if from us, to the effect that the day of the Lord has come.
(3) Let no one in any way deceive you, for it will not come unless the apostasy comes first, and the man of lawlessness is revealed, the son of destruction,
(4) who opposes and exalts himself above every so-called god or object of worship, so that he takes his seat in the temple of God, displaying himself as being God.
(5) Do you not remember that while I was still with you, I was telling you these things?
(6) And you know what restrains him now, so that in his time he will be revealed.
(7) For the mystery of lawlessness is already at work; only he who now restrains (restrains will restrain, NASB and NIV) will do so until he is taken out of the way.
(8) Then that lawless one will be revealed whom the Lord will slay with the breath of His mouth and bring to an end by the appearance of His coming;
(9) that is, the one whose coming is in accord with the activity of Satan, with all power and signs and false wonders,
(10) and with all the deception of wickedness for those who perish, because they did not receive the love of the truth so as to be saved.
(11) For this reason God will send upon them a deluding influence so that they will believe what is false,
(12) in order that they all may be judged who did not believe the truth, but took pleasure in wickedness.” NASB
When we understand that the corporate body of Christ - believers - is the "temple of God" we can begin to understand the "man of sin" found in 2nd Thessalonians.
2 Thessalonians 2:3-4 “Let no man deceive you by any means: for that day shall not come, except there come a departing first, and that man of sin be disclosed, even the son of perdition.
(4) Which is an adversary, and exalteth himself against all that is called God, or that is worshipped: so that he doth sit as God in the Temple of God, showing himself that he is God.”
The “man of sin” will “exalteth himself against all that is called God.”
John 1:1 “In the beginning was the Word, and that Word was with God, and that Word was God.”
Hence the “Temple of God” which exists in the Christian era - the corporate body of Christ - is composed of individuals. The "temple of God is in the human heart.
Consequently "he" that takes his seat in the human heart is not a human being but a spirit, the same "spirit of antichrist" mentioned in 1st and 2nd John.
It is much easier to detect Antichrists than “the son of perdition.”
Antichrist “denies that Jesus is the Christ;” he “denies the Father and the Son. Whoever denies the Son does not have the Father.” “…every spirit that does not confess Jesus is not from God…” “…those who do not acknowledge that Jesus Christ is come in the flesh. This is THE deceiver and the antichrist.”
Judas Iscariot was described as “the son of perdition” and was among the most trusted disciples. “Let no man deceive you by any means: for that day shall not come, except there come a falling away first…”
Apparently, “that man of sin” is a spirit in persons within the church “which is an adversary, and exalteth himself against all that is called God, or that is worshipped: so that he doth sit as God in the Temple of God, showing himself that he is God.”
Who is the “man of sin” if it isn't Satan?
I Timothy 4: 1-2 “Now the Spirit speaketh evidently, that in the latter times some shall depart from the faith, and shall give heed unto spirits of error, and doctrines of devils. (2) Which speak lies through hypocrisy, and have their consciences burned with an hot iron."
2 Timothy 4:3-4 “For the time will come when they will not suffer wholesome doctrine: but having their ears itching, shall after their own lusts get them an heap of teachers. (4) And shall turn their ears from the truth, and shall be given unto fables.”
It is currently fashionable to suggest that the rebuilding a physical “temple of God” on the temple mount in Jerusalem is scriptural. To believe so would require Christians to believe that the finished work of redemption of Jesus Christ is incomplete and unfinished.
Jesus gave the Pharisees the following warning.
Mark 7:13 “Making the word of God of none authority, by your tradition which ye have ordained; and ye do many such things.”
Hebrews 8:13 “In that he saith a New Testament, he hath abrogated the old: now that which is disannulled and waxed old, is ready to vanish away.”
Romans 1:16 “For I am not ashamed of the Gospel of Christ; for it is the power of God unto salvation to everyone that believeth, to the Jew first and also to the Grecian.”