Matthew 5:35

Next, to lead them farther away from swearing by God, He saith, "Neither by Heaven, for it is God's throne, nor by the earth, for it is the footstool of His feet; nor by Jerusalem, for it is the city of the great King:" still speaking out of the prophetical writings, and signifying Himself not to be opposed to the ancients. This was because they had a custom of swearing by these objects, and He intimates this custom near the end of His Gospel. But mark, I pray thee, on what ground He magnifies the elements; not from their own nature, but from God's relation to them, such as it had been in condescension declared. For because the tyranny of idolatry was great, that the elements might not be thought worthy of honor for their own sake, He hath assigned this cause, which we have mentioned, which again would pass on to the glory of God. For He neither said, "because Heaven is beautiful and great," nor, "because earth is profitable;" but "because the one is God's throne, the other His footstool;" on every side urging them on towards their Lord.

Lastly, consider that the Saviour does not here forbid to swear by God, but by the Heaven, the Earth, by Jerusalem, by a man's head. For this evil practice of swearing by the elements the Jews had always, and are thereof often accused in the prophetic writing's. For he who swears, shows either reverence or love for that by which he swears. Thus when the Jews swore by the Angels, by the city of Jerusalem, by the temple and the elements, they paid to the creature the honour and worship belonging to God; for it is commanded in the Law that we should not swear but by the Lord our God.