Deuteronomy 6:13

Thou shalt fear the Lord thy God, and serve him. — Literally, Jehovah thy God thou shalt fear, and him shalt thou serve: i.e., Him only, as translated by the LXX., and cited by our Lord in His temptation. It is remarkable that all His answers to the tempter were taken not only from Deuteronomy, but from one and the same portion of Deuteronomy — Deuteronomy 5-10 inclusive — the portion which applies the principles of the Decalogue to Israel’s life.

And shalt swear by his name. — Comp. Exodus 23:13. “Make no mention of the name of other gods.” The principle was not unknown to the patriarchs. Laban appealed to the “God of Nahor,” but “Jacob sware by the fear of his father Isaac” (Genesis 31:53). (Comp. Jeremiah 5:7 : “Thy children have forsaken me, and sworn by... no-gods.”)