This man is so egotistical that he becomes the enemy of everything worshipped as god. He even sets himself up in the Temple of God. Why does he do this? To receive the recognition that he feels is his due. There are several clues here that help us to identify this person further. The first is that he exalts himself above every so-called god. Notice what this same apostle says in another place:
For even if there are so-called gods, whether in heaven or on earth (as there are many gods, and many lords), yet for us [Christians] there is only one God, the Father, of whom are all things, and we for Him; and one Lord Jesus Christ, through whom are all things, and through whom we live. (I Corinthians 8:5-6)
What we have clarified, when compared to II Thessalonians 2:4, is that there is, in reality, only one God. But there are many so-called gods—that is, demons or inanimate objects that people worship as gods. The man of sin exalts himself over the true God and the so-called gods. When this is compared with the last clause, "he sits in the temple of God, showing himself that he is God," there is no other honest conclusion that can be reached but that this is the Temple in Jerusalem.
The apostle is using language that is in no way figurative. Everything that has been given so far, as part of this sign, is literal. The man is literal, the falling away is literal, and are we now asked to suspend that literality and believe that the Temple is suddenly figurative? That the Temple is the church?
The temple is located in Jerusalem, which is the focal point of three religions: Christianity, Judaism, and Islam. This means the man of sin will play a prominent role in the city of Jerusalem—which is real and literal—in the future, which emphasizes that these are events of worldwide significance. The contrast the apostle makes is between this man, who exalts himself against so-called gods, and the wretched blasphemy of comparing himself as greater than the Reality, God Himself.