Chapter 25 — Part III: The Decalogue — The Second Commandment
Section 37. When our Lord says: Let your speech be "yea, yea"; "no, no," He evidently forbids the habit of swearing in familiar conversation and on trivial matters. He therefore admonishes us particularly against being too ready and willing to swear; and this should be carefully explained and impressed on the minds of the faithful. That countless evils grow out of the unrestrained habit of swearing is proved by the evidence of Scripture, and the testimony of the most holy Fathers. Thus we read in Ecclesiasticus: Let not thy mouth be accustomed to swearing, for in it there are many falls; and again: A man that sweareth much shall be filled with iniquity, and a scourge shall not depart from his house. In the works of St. Basil and St. Augustine against lying, much more can be found on this subject.