4who opposes and exalts himself against every so-called god or object of worship, so that he takes his seat in the temple of God, proclaiming himself to be God.
Is proclaiming himself to be God an additional act/declaration OR is Paul saying:
the act of taking his seat in the temple of God is equivalent to proclaiming himself to be God
The pertinent word here is ἀποδείκνυμι (apodeiknumi) from the root word δείκνυμι, to show. ἀποδείκνυμι is a strengthened form that offers proof of something on the basis of associated evidence.
Thus, the force of Paul's prophecy is that the act of sitting in God's temple is the assertion and proof of the false claim to divinity.
Ellicott observes this:
Shewing himself.—Or, thus showing himself off. It does not mean that he makes any attempt to prove that he is God; the word only carries on the pictorial representation of the Man of Sin enthroning himself upon the Mercy Seat, and by that act of session parading his pretended divinity.
The Pulpit commentary is similar:
Showing - exhibiting - himself that he is God. His sitting in the temple of God was an assertion of his divinity; he claimed to be regarded and worshipped as God. This was the crowning act of his impiety;
