But I say unto you, Swear not at all; neither by heaven; for it is God's throne:

People's New Testament

5:34,35 Swear not at all. Christ does not forbid judicial oaths. Note, (1) God sometimes swears by himself (Ge 22:16,17); (2) Jesus made oath before the Sanhedrin (Mt 26:63); (3) Paul made oath to the Corinthians (2Co 1:23). He does forbid all profanity and idle oaths, such as were common among the Jews, and still so defile the mouths of men.

Neither by heaven. The Jews held that it was impious to swear by the name of God, but that one could swear by heaven, by the earth, by Jerusalem. One was God's throne, the second his footstool, Jerusalem the city of the Messiah King, all too holy for profanation.

Matthew Henry's Concise Commentary

5:33-37 There is no reason to consider that solemn oaths in a court of justice, or on other proper occasions, are wrong, provided they are taken with due reverence. But all oaths taken without necessity, or in common conversation, must be sinful, as well as all those expressions which are appeals to God, though persons think thereby to evade the guilt of swearing. The worse men are, the less they are bound by oaths; the better they are, the less there is need for them. Our Lord does not enjoin the precise terms wherein we are to affirm or deny, but such a constant regard to truth as would render oaths unnecessary.