Thou shalt fear the LORD thy God; him shalt thou serve, and to him shalt thou cleave, and (i) swear by his name.
(i) Read (Deuteronomy 6:13).

Thou shalt fear the Lord thy God,.... Which includes the whole worship of him, external and internal:
him shalt thou serve; heartily and sincerely, according to his revealed will, and him only:
and to him shall thou cleave; and not turn aside to follow other gods, and worship them:
and swear by his name; and his only, whenever it is necessary to take an oath, which should not be done rashly or on trivial accounts; and never by any creature, but by the living God; see Deuteronomy 6:13.

After laying down the fundamental condition of a proper relation towards God, Moses describes the fear of God, i.e., true reverence of God, in its threefold manifestation, in deed (serving God), in heart (cleaving to Him; cf. Deuteronomy 4:4), and with the mouth (swearing by His name; cf. Deuteronomy 6:13). Such reverence as this Israel owed to its God; for "He is thy praise, and He is thy God" (Deuteronomy 10:21). He has given thee strong inducements to praise. By the great and terrible things which thine eyes have seen, He has manifested Himself as God to thee. "Terrible things" are those acts of divine omnipotence, which fill men with fear and trembling at the majesty of the Almighty (cf. Exodus 15:11). אתּך עשׂה, "done with thee," i.e., shown to thee (את in the sense of practical help).

To him shalt thou cleave - With firm confidence, true affection, and constant obedience.