WCF 22.2.
The name of God only is that by which men ought to swear, and therein it is to be used with all holy fear and reverence. (Deut. 6:13) Therefore, to swear vainly, or rashly, by that glorious and dreadful Name; or, to swear at all by any other thing, is sinful, and to be abhorred. (Exod. 20:7, Jer. 5:7, Matt. 5:34, 37, James 5:12) Yet, as in matters of weight and moment, an oath is warranted by the Word of God, under the new testament as well as under the old; (Heb. 6:16, 2 Cor. 1:23, Isa. 65:16) so a lawful oath, being imposed by lawful authority, in such matters, ought to be taken. (1 Kings 8:31, Neh. 13:25, Ezra 10:5)