Adam Clarke

Swear unto me by the Lord - This is a farther proof that this woman had received considerable instruction in the Jewish faith; she acknowledged the true God by his essential character Jehovah; and knew that an oath in his name was the deepest and most solemn obligation under which a Jew could possibly come. Does not this also refer to the command of God, Thou shalt fear the Lord, and shalt swear by his name? See the note on Deu 6:13.