James records Jesus to have taught this, as does Matthew. The teaching clearly runs cross-current in his own day, as well as in ours. According to the NT criteriological standard known as the “criterion of counterproductive features,” this makes it quite likely that Jesus did in fact teach this, and that James learned it from him as well. Many commentators regard the epistle of James as a kind of brief commentary on selected aspects of the “Sermon on the Mount” (Matthew 5 – 7), which is about the “kingdom of God” (in the Lukan counterpart) and appropriate behaviors and practices conducive to its citizenship.
This would also follow from several other truisms taught in the Bible elsewhere:
1. Oaths involve imposing a self-curse, a (divinely enforced) curse against one’s self should one fail to keep one’s oath-based obligations. This defies the command to “love your neighbor AS yourself,” where “love your neighbor” means “bless [those who curse you] and do not curse” and “do not condemn, or you too will be condemned; and “with the measure you use, it will be measured [back] to you.” James teaches this as well.
2. Oaths are unnecessary speech, and simply reiterate what is already promised. Only the promise is necessary, not the oath. This means that all oaths are frivolous and represent false religion.
3. Oaths invoke God’s name and often try to get Him to affirm propositional content that He Himself would not allow. This abuses the divine name. Marriage is by solemn promise, not by invoking God’s name illegally (oath). Some will protest that this destroys the distinction between the holy and the common. But this merely begs the question in favor of the categorical distinction not taught in the early chapters of Genesis. Adam and Eve were not married by oath. This did not make marriage less than holy, since God Himself instituted it (THAT is what makes it holy; God created it; its holiness, or special standing is also of the light of nature and the procreation of the human race).
4. This means that sections of the Scripture affirming that God makes oaths are either not relevant as to whether we may do the same (It does not necessarily follow that if God may do some action X that we may do some action X), or else they are not properly original to the text.