Deuteronomy 6:13

Thou shalt fear the Lord thy God - Thou shalt respect and reverence him as thy Lawgiver and Judge; as thy Creator, Preserver, and the sole object of thy religious adoration.

And serve him - Our blessed Lord, in Matthew 4:10; Luke 4:8, quotes these words thus: And him Only (αυτῳ μονῳ) shalt thou serve. It appears, therefore, that לבדו lebaddo was anciently in the Hebrew text, as it was and is in the Septuagint, (αυτῳ μονῳ), from which our Lord quoted it. The Coptic preserves the same reading; so do also the Vulgate, (illi soli), and the Anglo-Saxon. Dr. Kennicott argues, that without the word only the text would not have been conclusive for the purpose for which our Lord advanced it; for as we learn from Scripture that some men worshipped false gods in conjunction with the true, the quotation here would not have been full to the point without this exclusive word. It may be proper to observe that the omitted word לבדו lebaddo, retained in the above versions, does not exist in the Hebrew printed text, nor in any MS. hitherto discovered.

Shalt swear by his name - תשבע tishshabea, from שבע shaba, he was full, satisfied, or gave that which was full or satisfactory. Hence an oath and swearing, because appealing to God, and taking him for witness in any case of promise, etc., gave full and sufficient security for the performance; and if done in evidence, or to the truth of any particular fact, it gave full security for the truth of that evidence. An oath, therefore, is an appeal to God, who knows all things, of the truth of the matter in question: and when a religious man takes such an oath, he gives full and reasonable satisfaction that the thing is so, as stated; for it is ever to be presumed that no man, unless in a state of the deepest degradation, would make such an appeal falsely, for this would imply an attempt to make God a party in the deception.

The command "to swear by His Name" is not inconsistent with the Lord's injunction Matthew 5:34, "Swear not at all." Moses refers to legal swearing, our Lord to swearing in common conversation. It is not the purpose of Moses to encourage the practice of taking oaths, but to forbid that, when taken, they should be taken in any other name than that of Israel's God. The oath involves an invocation of Deity, and so a solemn recognition of Him whose Name is made use of in it. Hence, it comes especially within the scope of the commandment Moses is enforcing.

6:13 Shalt swear by his name - When thou hast a call and just cause to swear, not by idols, or any creatures.