When Jesus forbids swearing by heaven, because “it is God's throne,” and by the earth, because “it is his footstool,” the inference is obvious that, for still stronger reasons, all direct swearing by God himself is prohibited. The word μήτε, which introduces the oaths by inferior objects specified in the text under discussion, not infrequently corresponds to our phrase not even. With this sense of μήτε, the passage would be rendered, “But I say unto you, Swear not at all, not even by heaven,” etc.
I find that some writers on this subject quote in vindication of oaths on solemn occasions the instances in the Scriptures in which God is said to have sworn by Himself. The reply is obvious, that no being can swear by himself, the essential significance of an oath being an appeal to some being or object other than one's self. Because God “can swear by no greater,” it is certain that when this phraseology is used concerning Him, it is employed figuratively, to aid the poverty of human conceptions, and to express the certainty of his promise by the strongest terms which human language affords. In like manner, God is said by the sacred writers to repent of intended retribution to evil-doers, not that infinite justice and love can change in thought, plan, or purpose, but because a change of disposition and feeling is wont to precede human clemency to evil-doers.