TREATISE II: On the Second Precept of the Decalogue

Thou shalt not take the name of the Lord thy God in vain.
(Exod., 20, 7).

593. As through the first precept fidelity toward God is chiefly prescribed, or proper submission to him, so through the second precept reverence is chiefly prescribed, or that other part of worship by which we are commanded to commit nothing injurious against God, and to honor the name, that is, the excellent power, of God (St. Thomas, Op. de praec. Decal., c. 13).

Now the divine name is honored: 1. by confession and praise; the contrary of which is vain assumption, and much more blasphemy; 2. by oath, for by swearing we profess God to be the first truth, likewise by adjuration, for by this we profess the power of the divine name; the opposite of these is a false, unjust, and indiscreet oath, as also indiscreet adjuration; 3. by vow, for by vowing we profess God to be most worthy of every service; to this is opposed an indiscreet vow and violation of a vow.

Hence there must be treated of three things, namely: 1. of the praise of God, and of irreverence toward his holy name; 2. of oath and adjuration; 3. of vow.