We have been united as follows concerning the oath. The oath is a confirmation among those who are quarreling or making promises. In the law it is commanded that it should be done only in the name of God, truthfully and not falsely. Christ, who teaches the perfection of the law, forbids His [followers] all swearing, whether true or false; neither by heaven nor by earth, neither by Jerusalem nor by our head; and that for the reason which He goes on to give: "For you cannot make one hair white or black." You see, thereby all swearing is forbidden. We cannot perform what is promised in the swearing, for we are not able to change the smallest part of ourselves (Mt. 5:34-37).
Now there are some who do not believe the simple commandment of God and who say, "But God swore by Himself to Abraham, because He was God (as He promised him that He would do good to him and would be his God if he kept His commandments). Why then should I not swear if I promise something to someone?" The answer: hear what the Scripture says: "God, since he wished to prove overabundantly to the heirs of His promise that His will did not change, inserted an oath so that by two immutable things we might have a stronger consolation (for it is impossible that God should lie") (Heb 6:7 ff.). Notice the meaning of the passage: God has the power to do what He forbids you, for everything is possible to Him. God swore an oath to Abraham, Scripture says, in order to prove that His counsel is immutable. That means: no one can withstand and thwart His will; thus He can keep His oath. But we cannot, as Christ said above, hold or perform our oath, therefore we should not swear.
Others say that swearing cannot be forbidden by God in the New Testament when it was commanded in the Old, but that it is forbidden only to swear by heaven, earth, Jerusalem, and our head. Answer: hear the Scripture. He who swears by heaven, swears by God's throne and by Him who sits thereon (Mt. 5:35). Observe: swearing by heaven is forbidden, which is only God's throne; how much more is it forbidden to swear by God Himself. You blind fools, what is greater, the throne or He who sits upon it?
Others say, if it is then wrong to use God for truth, then the apostles Peter and Paul also swore. Answer: Peter and Paul only testify to that which God promised Abraham, whom we long after have received. But when one testifies, one testifies concerning that which is present, whether it be good or evil. Thus Simeon spoke of Christ to Mary and testified: "Behold: this one is ordained for the falling and rising of many in Israel and to be a sign which will be spoken against" (Lk 2:34).
Christ taught us similarly when He says: Your speech shall be yea, yea; and nay, nay; for what is more than that comes of evil. He says, your speech or your word shall be yes and no, so that no one might understand that He had permitted it. Christ is simply yea and nay, and all those who seek Him simply will understand His Word. Amen.