He will oppose and exalt himself above every so-called god or object of worship. So he will seat himself in the temple of God, proclaiming himself to be God.
KJV
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.
ASV
he that opposeth and exalteth himself against all that is called God or that is worshipped; so that he sitteth in the temple of God, setting himself forth as God.