John Gill Commentary Deuteronomy 10:20

"Thou shalt fear Jehovah thy God; him shalt thou serve; and to him shalt thou cleave, and by his name shalt thou swear." — Deuteronomy 10:20 (ASV)

You shall fear the Lord your God Which includes the whole worship of him, external and internal:

him shall you serve ; heartily and sincerely, according to his revealed will, and him only:

and to him shall you 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) .