Chapter 25 — Part III: The Decalogue — The Second Commandment
Section 36. It cannot be asserted that these words condemn oaths universally and under all circumstances, since we have already seen that the Apostles and our Lord Himself made frequent use of them. The object of our Lord was rather to reprove the perverse opinion of the Jews, who had persuaded themselves that the only thing to be avoided in an oath was a lie. Hence in matters the most trivial and unimportant they did not hesitate to make frequent use of oaths, and to exact them from others. This practice the Redeemer condemns and reprobates, and teaches that an oath is never to be taken unless necessity require it. For oaths have been instituted on account of human frailty. They are really the outcome of evil, being a sign either of the inconstancy of him who takes them, or of the obstinacy of him who refuses to believe without them. However, an oath can be justified by necessity.