Book Five — On the Decalogue — Treating of the Second Commandment:

"Thou shalt not take the name of the Lord thy God in vain: for the Lord will not hold him guiltless that shall take the name of the Lord his God in vain." — Exodus 20:7

The first commandment of the Decalogue has supplied us with an ample subject for disputation. The second, however, presents a subject for discussion not, indeed, so vast, but far more sorrowful on account of the depravity of Christians. For in the commentary on that commandment, which I undertake to treat with a fuller pen, lies that oath which has always been held in the highest veneration, and in great esteem, by all nations, however wild, barbarous, and uncultivated. But, O the times, O the customs! Although the Christian religion, above all others, professes the sanctity of the oath, and teaches that it is to be employed with reverence, only in the most extreme circumstances, its conditions observed, and with the greatest caution, on solemn occasions only — nonetheless the abuse is so great, that very many Christians have nothing more frequent in their mouths, in the midst of professing this sound doctrine and evangelical teaching, than oaths, whether in ratifying pacts and contracts, or in the buying and selling of goods, now in games and amusements, now, finally, in every familiar conversation. This frequency of swearing, moreover, introduces such a habit of perjury, that it exposes the law and profession of Jesus Christ to the contempt, mockery, and reproach, indeed nearly to the peril, of all the infidel nations. For pagans cannot bring themselves to believe that Christians worship and love the true God with faith, hope, and charity, when they hear them everywhere holding his most sacred name in contempt, and abusing it in almost every word.

But what demands a still more abundant fount of tears is the teaching of certain Casuists, who do not recall the laws themselves, and the rules to be observed in the use of oaths, back to a just and holy custom of swearing; but, on the contrary, accommodate the rules of oaths to the depraved morals of perjurers — which I would not have understood in the sense that any of them approve the horrendous crime of perjury; but rather that, deceived by a false and cruel benignity, not a few of them, when they hear the holy name of God usurped in almost every utterance, strive, with a merciful intention and in good faith, to persuade the multitude that this is not truly an oath according to the sacred Scriptures and the holy Fathers, so as to withdraw very many from the danger of perjury — that swearing is not the formula of this or that particular nation, but a peculiar one. For the unbridled and unchecked license of many in employing the divine name without discrimination, judgment, and justice has given occasion to not a few submissive writers to write that men do not swear, although they affirm "By God" that what they utter is true — since either they have no intention of swearing, or this invocation "By God" is, among certain nations, reduced to a mere customary manner of speaking; or, by mental restrictions, the matter is withdrawn from the oath; or the oath is directed toward some other meaning than that which the one to whom it is sworn understands. Hence the frequency of Christians' swearing has driven not a few Casuists to coin lax opinions; and their opinions, in turn, more bold, have made Christians more reckless in the use of oaths; and finally, both on account of the laxity of the former's writing, and on account of the latter's vain usurpation of the divine name, the sacrilegious custom of perjury has struck such deep roots, and flourishes so obstinately and with such impunity everywhere, that it has already almost shed the enormity of its crime. Wherefore, before I proceed to untangling the difficulties of the oath, I will set before the eyes, according to custom, the errors of the heretics, and the propositions of certain Casuists, proscribed by the Church, and, partly, though not proscribed, nonetheless straying too far from evangelical honesty. To all these, however, I must first prefix what God really commands, and what he forbids, by this his second commandment. For very many things are contained in these words, "thou shalt take the name of the Lord thy God in vain," which, unless I expound beforehand, my discourse, being defined by no boundary, could not be consistent with itself everywhere, nor proceed by a straight path. I will, then, first disclose the various meanings of the words by which God expressed his second commandment; then I will proceed to handling the proposed subject.