Albert Barnes Commentary Matthew 5:34-35

"but I say unto you, swear not at all; neither by the heaven, for it is the throne of God; nor by the earth, for it is the footstool of his feet; nor by Jerusalem, for it is the city of the great King." — Matthew 5:34-35 (ASV)

Swear not at all. That is, in the manner which he proceeds to specify. Swear not in any of the common and profane ways customary at that time.

By Heaven; for it is God's throne. To swear by that was, if it meant anything, to swear by Him that sitteth thereon (Matthew 23:22).

The earth; for it is his footstool. Swearing by that, therefore, is really swearing by God. Or perhaps it means,

1. we have no right to pledge, or swear by, what belongs to God; and,
2. that oaths by inanimate objects are meaningless and wicked. If they are real oaths, they are by a living Being, who has power to take vengeance. A footstool is that on which the feet rest when sitting. The term is applied to the earth, to denote how lowly and humble an object it is when compared with God.

Jerusalem (Matthew 2:1).

City of the great King. That is, of God; called the great King because he was the King of the Israelites, and Jerusalem was the capital of the nation, and the place where he was especially honoured as King.