The throne of God - Heaven is his throne, Matthew 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.
Related Commentaries of Matthew 23:22
Matthew 23:22
22 And he that shall swear by heaven, sweareth by the throne of God, and by him that sitteth thereon.