Deuteronomy 6:13

Ver. 13. Thou shalt—swear by his name— An oath is a solemn act of religious worship; so, to swear by any other name than that of the true God, is to recognize another god as the true one, and to fall into a polytheism. Hence it follows, that the words, thou shalt swear by his name, are synonymous with thou shalt profess the true religion; and, in divers passages of the Old Testament, the worshippers of the true God are especially characterised as those who swear by the name of the Lord. God himself, foretelling, by the mouth of Isaiah, the conversion of the Gentiles, does it in these words; unto me every knee shall bow, and every tongue shall swear, chap. Isa 45:23 which St. Paul explains, every tongue shall confess to God. Romans 14:11. See also Isaiah 19:18; Isaiah 65:16. Jeremiah 4:1-2; Jeremiah 5:7. This appears to be the true meaning of the passage: many, however, understand it, that in taking an oath they were to swear only by Jehovah, and never by any of the gods of the Gentiles. See Joshua 23:7-8.