The question of the lawfulness of oaths has oftentimes perplexed alike the doctors of the Church and its simpler hearers of God’s word. The text, taken as it stands, would support the views of the Essenes, and many of the Paulicians, and other ancient sectaries. With equal force it might be urged by the followers of Peter Waldo, or the Unitas Fratrum (the Moravians), or the Society of Friends.
Swear not. — The words are put quite distinctly in Greek and English — neither by the heaven, nor by the earth. And it sounds like special pleading, worthy of a rabbi, to hear such a divine as Huther say that “swearing by the name of God is not mentioned,” nor accordingly is such an oath prohibited. “We must not imagine,” he continues (and his argument had best be fairly given), “that this is included in the last member of the clause, the Apostle evidently intending by it (i.e., ‘neither by any other oath’) to point only at certain formulæ, of which several are mentioned in Matthew 5:34-37. Had he intended to forbid swearing by the name of God he would most certainly have mentioned it expressly; for not only is it in the Law, in contradistinction to other oaths, commanded (see Deuteronomy 6:13; Deuteronomy 10:20; Psalms 63:11), but in the prophets is announced as a token of the future turning of men to God” (Isaiah 65:16; Jeremiah 12:16; Jeremiah 23:7-8). There were, we learn, many subtle distinctions in Jewish oaths; and the unlucky foreigner who trusted in an apparently firm one, too often found out his mistake. Certainly all such subterfuges are utterly condemned; and further, every word which breaks the letter or spirit of God’s Third Commandment. As to the higher judicial forms of oaths, remembering that our Lord answered such before Caiaphas (Matthew 26:63-64), we can fearlessly conclude, with the 39th Article of Religion, that “a man may swear, when the magistrate requireth, in a cause of faith and charity, so it be done according to the prophet’s teaching — in justice, judgment, and truth.”
Let your yea be yea... — Your word be as your bond, needing no strengthening by any invocation of God, or holy things, “lest ye fall into judgment” — not “condemnation,” though certainly such might follow.