Albert Barnes on Matthew 23:22

The throne of God. Heaven is his throne, Mt 5:34. It is so called as being the place where he sits in glory. Jesus says, here, that all who swear at all, do in fact swear by God, or the oath is good for nothing. To swear by an altar, a gift, or a temple, is of no force, unless it be meant to appeal to God himself. The essential thing in an oath is calling God to witness our sincerity. If a real oath is taken, therefore, God is appealed to. If not, it is foolish and wicked to swear by anything else.

(h) "throne of God" Ps 11:4, Isa 66:1, Mt 5:34