John Calvin 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)

And you shall not swear by my name falsely. Although Moses is addressing the duties of the Second Table, and had previously forbidden people to deal fraudulently with their neighbors, he still adds this sentence as confirmation. It may, however, be inferred from the second clause of the verse that He directly focused on the glory of God when He says, You shall not profane the name of your God. For intense greed for gain causes the avaricious and rapacious man not only to defraud others, but also to become insolent toward God Himself.

Moses, therefore, although he is explicitly condemning the falsehood and deceit by which our neighbors are injured, at the same time takes the opportunity to declare that we must be careful, lest, while covetousness impels us to do wrong, injury should be done not only to other people but to God Himself as well. The word used here, however, is not שוא, shau, as before, but שקר, sheker, which properly means deceitfulness. Therefore, I have said that it commands us to be careful that no one by his perjury should do any injury to his neighbor. Nevertheless, this prohibition directly refers to the Third Commandment, since Moses especially insists on this point: that God’s name is profaned by perjury. Thus, he not only instills integrity, but also considers religion, so that God’s majesty may not be violated.

The expression is noteworthy, You shall not pollute the name of God, because God, who is eternal and immutable truth, cannot be more gravely insulted than by being summoned as a witness to falsehood, which is certainly a shameful and wicked pollution.

This was not regarded by the heathen, who, although they pretended to reverence God’s name in their oaths, yet did not hesitate to deceive if the one to whom they had made a promise deserved it. Thyestes in the poet says, “I never have pledged my faith, nor do I pledge it to any faithless person;”³¹⁰ since his brother was a villain, he considered that he was under no valid obligation to him. This is as if God’s majesty were dependent upon what people deserve, so that it was permissible to call Him to witness while we act deceitfully.

Let this, then, be our firm conclusion: that in our oaths God must be regarded first, whose holy name is more precious than a hundred worlds.

³¹⁰ Cic. de Off. 3 28, 29. “Deinde illud etiam apud Actium Fregisti fidem. Neque dedi neque do infideli cuiquam, quanquam ab impio rege dicitur, luculente tamen dicitur. “Nam illud quidem, Neque dedi, neque do fidem in fideli cuiquam, idcirco recte a poeta; quia, cum tractaretur Atreus, personae serviendum fuit.”.”