Genesis 4:15

Therefore. It is not entirely clear what antecedent idea is intended by this word. The RSV, following the LXX, the Syriac, and the Vulgate, renders it, “Not so!” In other words, to Cain’s declaration, “Every one that findeth me shall slay me,” God replied, “Not so!”

Sevenfold. This implies a most severe penalty upon anyone murdering Cain (see Lev. 26:18, 21, 24, 28; Ps. 79:12; Prov. 6:31). Special protection was granted him in harmony with the principle, “Vengeance is mine; I will repay, saith the Lord” (Rom. 12:19). The tares must grow together with the wheat; the fruits of sin must be permitted to reach maturity, in order that the character of its seed may be manifest. The lives of Cain and his descendants were to be exhibits of the outworking of sin in rational beings (PP 78).

A mark. Some commentators have seen in this mark an outward sign attached to Cain’s person, whereas others believe that he received a sign from God as a divine pledge that nothing would endanger his life. Whatever it was, it was not a sign of God’s forgiveness but only of temporal protection.