You shall fear the LORD your God, and serve him, and shall swear by his name.
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Augustine of Hippo
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. .
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. .
Only. This is omitted in Hebrew; but the Septuagint and Jesus Christ retain it, (Matthew iv. 10,) as the sense requires. You cannot serve God and mammon, Luke xvi. 13. (Calmet)
Name, and not by that of idols, whenever you may be authorized to take an oath. (Haydock)
To swear by any other, is to acknowledge him in some sort for a god. When we take an oath on proper occasions, and with due respect and caution, we perform an act of religion. (Calmet)