What did Melanchthon mean when he said a conscience terrified by the law can find comfort in a promise connected to good works (Ap. 3:151-155)? The answer is not immediately obvious. Following David Scaer in seeing a third use of the law that is distinct from its first two uses can help here. Since the same word "law" can mean different things even within the sixth article of the Formula of Concord, let me clarify. By "law," I here mean the unchanging will of God. I will use the same word in a more narrow sense shortly.
Melanchthon said Christ often attaches promises to good works not only to destroy Epicurean delusions, but also to offer a variety of signs and testimonies for the terrified conscience. A key to seeing how Melanchthon could find comfort for terrified consciences in those promises lies in properly distinguishing the second and third uses of the law, that is, between the law in the narrow sense and the fruit of faith, "against which there is no law."
The terrified conscience of Luther turned not only to the promise that those who believe and are baptized will be saved, but also to the promise that those who forgive are forgiven:
But there is here attached a necessary, yet consolatory addition: As we forgive. He has promised that we shall be sure that everything is forgiven and pardoned, yet in the manner that we also forgive our neighbor. For just as we daily sin much against God, and yet He forgives everything through grace, so we, too, must ever forgive our neighbor who does us injury, violence, and wrong, shows malice toward us, etc. If, therefore, you do not forgive, then do not think that God forgives you; but if you forgive, you have this consolation and assurance, that you are forgiven in heaven, not on account of your forgiving, for God forgives freely and without condition, out of pure grace, because He has so promised, as the Gospel teaches, but in order that He may set this up for our confirmation and assurance for a sign alongside of the promise which accords with this prayer, Luke 6:37: Forgive, and ye shall be forgiven. Therefore Christ also repeats it soon after the Lord's Prayer, and says, Matt. 6:14: For if ye forgive men their trespasses, your heavenly Father will also forgive you, etc. This sign is therefore attached to this petition, that, when we pray, we remember the promise and reflect thus: Dear Father, for this reason I come and pray Thee to forgive me, not that I can make satisfaction, or can merit anything by my works, but because Thou hast promised and attached the seal thereto that I should be as sure as though I had absolution pronounced by Thyself. For as much as Baptism and the Lord's Supper, appointed as external signs, effect, so much also this sign can effect to confirm our consciences and cause them to rejoice. And it is especially given for this purpose, that we might use and practise it every hour, as a thing that we have with us at all times. This is not Presbyterianism's uncertain "inward evidence of those graces unto which these promises are made", but the very certainty of the gospel: "...if you forgive, you have this consolation and assurance, that you are forgiven in heaven, not on account of your forgiving, ... Thou hast promised and attached the seal thereto that I should be as sure as though I had absolution pronounced by Thyself."
Luther indeed found comfort in the promise of Mark 16. He also found comfort in the promise of Luke 6:37 and Matt. 6:14. He needed manifold consolation. After all, he subscribed to Melanchthon's statement that "we have need of external signs of so great a promise, because a conscience full of fear has need of manifold consolation. As, therefore, Baptism and the Lord's Supper are signs that continually admonish, cheer, and encourage desponding minds to believe the more firmly that their sins are forgiven, so the same promise is written and portrayed in good works, in order that these works may admonish us to believe the more firmly."