John Gill Reformed Baptist @doctorgill

And he that shall swear by heaven,.... As the Jews were wont to do in common, but did not look upon such an oath as obligatory on them; See Gill on Mat 5:34, though such an one sweareth by the throne of God; for heaven is God's throne, where he sits, and, in an eminent manner, displays the glory of his majesty: and by him...

Albert Barnes Presbyterian @notesbybarnes

Verse 21. Him that dwelleth therein. That is, God. The temple was his house, his dwelling. In the first, or Solomon's temple, he dwelt between the cherubims, in the most holy place. He manifested himself there by a visible symbol, in the form of a cloud resting on the mercy-seat, . (g) "him that dwelleth"