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.

And he that shall swear by heaven, sweareth by the (s) throne of God, and by him that sitteth thereon.
(s) If heaven is God's throne, than he is without doubt above this entire world.

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 Matthew 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 that sitteth thereon, by God himself. Thus swearing by anything that has any relation to God, is implicitly swearing by him; and therefore ought to be considered as binding, as if he was expressed in it; since an appeal cannot be made to things inanimate, nor indeed to any creature, but to God, the searcher of hearts.