Walton, Matthews, & Chavalas,
The IVP Bible Background Commentary: Old Testament:
"Since oaths were considered powerful and effective, the utterance of oaths would demonstrate which deity was truly considered powerful. Though inheriting the cities, homes and farms of the Canaanites, the Israelites are not to inherit the gods that had been associated with protecting these cities and providing fertility to this land. One of the ways to demonstrate their rejection of those gods is to refuse to attribute power to them through oaths."
Moshe Weinfeld, The Anchor Bible Vol. 5, Deuteronomy 1-11, p. 345:
"Loyalty to YHWH means to recognize Him alone as the arbitrating power in all man’s dealings with His fellows and especially in one’s oaths in which the name of one’s god is invoked. He who swears by YHWH is a true worshiper."
An oath was a religious formula by which the divine forces of the universe are invoked as witnesses, enforcers, or avengers in case of violation.” (Betz, 260). An oath combined God's name and character to the promise. To break an oath, then, smacked of blasphemy.
Jesus forbade oaths (Mt. 5.33-37) because anyone who truly belonged to His kingdom didn't need them, since they would automatically be truth tellers, have integrity, and pursue righteousness. Richard Hays, in
The Moral Vision of the New Testament explains that "The teachings of the rest of the sermon, after the Beatitudes, specify the character of a community that seeks to embody this eschatological vision of God’s righteousness."
F.F. Bruce, in
The Hard Sayings of Jesus, pp. 66-67, writes,
"Perjury is a serious offense in any law code. To swear an oath falsely in the name of God was a sin not only against the name but against the very person of God. The followers of Jesus should be known as men and women of their word. If they are known to have a scrupulous regard for truth, then what they way will be accepted without the support of any oath."