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
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