John the Apostle
Revelation 13:6BSB·traditional attribution

And the beast opened its mouth to speak blasphemies against God and to slander His name and His tabernacle—those who dwell in heaven.

Matthew Henry Presbyterian @wholebiblehenry

We have here an account of the rise, figure, and progress of the first beast; and observe, 1. From what situation the apostle saw this monster. He seemed to himself to stand upon the sea-shore, though it is probable he was still in a rapture; but he took himself to be in the island Patmos, but whether in the body or out of the body he could not tell. 2.

Commenting on Revelation 13:1-10

John Gill Reformed Baptist @doctorgill

And he opened his mouth in blasphemy against God,.... By sitting as God in the temple of God, showing himself that he is God; by suffering himself to be called by the name of God, God on earth, Lord God the pope, and to be worshipped as God; and by assuming infallibility, giving out that he cannot err, which only belongs to the God of...

Albert Barnes Presbyterian @notesbybarnes

Verse 6. And he opened his mouth in blasphemy against God, to blaspheme his name. By his own arrogant claims; by his assumed authority in matters of conscience; by setting aside the Divine authority; and by impious declarations in derogation of the Divine claims. . And his tabernacle. Literally, "his tent"--σκηνην.