223. There are two kinds of promises, or a twofold promise, as we have often explained. There are the legal promises, if I may so call them, which depend, as it were, upon our own works, such as the following: "If ye be willing and obedient, ye shall eat the good of the land," Is 1, 19. Again, I am God, showing mercy unto thousands of them that love me and keep my commandments, Ex 20, 6. And also above, in this case of Cain, "If thou doest well, shall not thy countenance be lifted up?" Gen 4, 7. And these legal promises have for the most part their corresponding threats attached to them.
But the other kind of promises are promises of grace, and with them no threats are joined. Such are the following: "Jehovah thy God will raise up unto thee a prophet from the midst of thee, of thy brethren, like unto me; unto him ye shall hearken," Deut 18, 15. Again, "I will put my law in their inward parts, in their heart will I write it; and I will be their God, and they shall be my people," Jer 31, 33. And again, "I will put enmity between thee and the woman," Gen 3, 15. Now, these promises depend not in any way upon our works, but absolutely and only upon the goodness and grace of God, because he was pleased to make those promises and to do what he thus promised. Just in the same way we have the promise of Baptism, of the Lord's Supper, and of the Keys, etc., in which God sets before us his good will and his mercy and his works.
224. Now, God gave no promise of the latter kind to Cain. He only said to him, Whosoever shall slay thee shall be punished sevenfold. But Adam had such a promise of grace made to him. And Cain, because he was the first-born, ought to have received that promise as an inheritance from his parents. That promise was the large and blessed promise of eternal glory, because by it the seed was promised which should bruise the serpent's head, and this without any work or merit of man. For that promise had no condition attached to it, such as, If thou shalt offer thy sacrifices, if thou shalt do good, etc.
225. If, therefore, you compare this promise of grace with the words God spake to Cain, the latter are as a mere crust held out to a beggar. For even Cain's life is not promised him absolutely. Nothing more is said than a threat pronounced against those who should slay him. God does not say positively, No man shall slay thee. He does not say, I will so overrule all others that no one shall slay thee. Had the words been thus spoken, Cain might have returned into the presence of God and of his parents. But a command only is given to men that they slay not Cain. If, therefore, the words spoken to Cain be at all considered as a promise, it is that kind of promise which, as we have before said, depends on the works and will of man. And yet, even such promise is by no means to be despised, for these legal promises often embrace most important things.
226. Thus, Augustine observes that God gave to the Romans their empire on account of their noble virtues. And in the same manner we find, even to this day, that the blessings of those nations which keep from murder, adultery, theft, etc., are greater than those of other nations in which these evils prevail. And yet, even governments which, as far as mere reason can succeed, are especially well established, possess nothing beyond these temporal promises.
227. The Church, however, possesses the promises of grace, even the eternal promises. And although Cain was left utterly destitute of these promises, yet it was a great favor that the temporal mercies were left him: that he was not immediately killed, that a wife was given him, that children were born unto him, that he built a city, that he cultivated the earth, that he fed his cattle and had possessions, and that he was not utterly ejected from the society and fellowship of men. For God could not only have deprived Cain of all these blessings, but he could have added pestilence, epilepsy, apoplexy, the stone, the gout, and any other disease. And yet there are men disposed curiously to argue in what manner God could possibly have multiplied the curse of Cain sevenfold on himself or on any other.
As God above deprives Cain of all the divine blessings, both spiritual—or those pertaining to the Church—and civil, so here he mitigates that sentence by commanding that no one shall slay Cain. But God does not promise at the same time that all men shall surely obey his command. Therefore Cain, even possessing this promise in reference to his body, is still a fugitive and a wanderer. And it might be that if he continued in his wickedness, he was liable to be slain at any moment; whereas, if he did well, he might live a long time. But nothing is promised him with certainty, for although these corporal or legal promises are great and important, yet they are positively uncertain and uncovenanted.