As the law teaches, “You shall love the Lord your God with your whole heart and your whole soul.” But the person who puts his entire hope in the Lord also praises with his whole heart. He does not put his trust in the transient consolations of the world, once he has trained himself on the Lord with total purity of mind.
Again, the devil taketh him up into an exceeding high mountain, and showeth him all the kingdoms of the world, and the glory of them; And saith unto him, All these things will I give thee, if thou wilt fall down and worship me. Then saith Jesus unto him, Get thee hence, Satan: for it is written, Thou shalt worship the Lord thy God, and him only shalt thou serve. [Deuteronomy 6:13] Then the devil leaveth him, and, behold, angels came and ministered unto him.
And the devil, taking him up into an high mountain, showed unto him all the kingdoms of the world in a moment of time. And the devil said unto him, All this power will I give thee, and the glory of them: for that is delivered unto me; and to whomsoever I will I give it. If thou therefore wilt worship me, all shall be thine. And Jesus answered and said unto him, Get thee behind me, Satan: for it is written, Thou shalt worship the Lord thy God, and him only shalt thou serve. [Deuteronomy 6:13]
What is to be said of justice in its relation to God? As the Lord says, “No man can serve two masters,” and the apostle rebukes those who serve the creature rather than the Creator, so had it not been said before in the Old Testament: “You shall adore the Lord your God and him only shall you serve”? But what need is there to say more about this here since the Scriptures are full of such texts? Justice then offers this rule of life to the lover we are describing: that he serve with gladness the Lord whom he loves, that is to say, the supreme good, the supreme wisdom, the supreme peace; and with respect to all other things, that he govern those which are subject to him and endeavor to subject all else to the same rule. This rule of life is confirmed, as we have shown, by the authority of both Testaments.
Whoever yields assent to the supreme authority of divine Scripture should first examine these words: “The Lord your God shall you adore, and him only shall you serve.” In Greek the expression used does not signify the service owed to human masters but that which is offered to God, called latreia. Thus idolatry is rightly condemned because the latreia which is due to the true God alone is offered to idols. It does not say, “You shall adore only the Lord your God,” but it says, “And him only shall you serve.” It used the word only with “you shall serve,” meaning, no doubt, that service which is called latreia. To this service belong temple, sacrifice, priest, and other like attributes.
[AD 585] Cassiodorus on Deuteronomy 6:10-19