God put a mark on Cain. This mark was one that would identify him as the murderer yet would also serve to protect him from the vengeance of others. It served as a sign of God’s promise that no vengeance would be taken on Cain. God put a 
rainbow in the sky after to flood as a sign that never again would the world be destroyed by a flood. God called Abraham to “mark” himself and all the males of his household as a sign that they too were part of the covenant that God was making with Abraham. It’s recorded in Genesis 17:10-11. It says, “This is my covenant, which you shall keep, between me and you and your offspring after you: Every male among you shall be circumcised. You shall be circumcised in the flesh of your foreskins, and it shall be a sign of the covenant between me and you.” The mark would be the sign that they are God’s people.
The “mark” was a guarantee. In Ephesians 1:14 Paul says, “The Spirit is God’s guarantee that he will give us the inheritance he promised and that he has purchased us to be his own people.” When Paul addresses his first letter to the believers in Corinth he says (1 Corinthians 1:2), “To the church of God that is in Corinth, to those sanctified in Christ Jesus, called to be saints together with all those who in every place call upon the name of our Lord Jesus Christ, both their Lord and ours.” In his commentary on this passage Richard Hays says, “This does not mean that the Corinthians have some special vocation that sets them apart from other Christians; rather, they—along with all other Christians—are set apart from a confused and perishing world, marked by God as God’s people. Paul regards all the members of all his churches as “the saints,” the elect of God.”
In Ephesians 4:30, Paul exhorts his readers not to grieve the Holy Spirit. It’s possible for true believers to live contrary to their calling as saints, but that does not jeopardize their status as saints. He exhorts believers to live lives worthy of their calling, not by threatening to remove them from God’s favor, but by reminding them of their secure status. He says, “Remember, he has identified you as his own, guaranteeing that you will be saved on the day of redemption.” Louis Berkhoff says, “…the fact that they are said to be sealed unto the day of redemption clearly indicates that the sealing of God secures their safety that they are thereby rendered sure of their final salvation.” John Stott explains the significance of this for us. He writes, “What the Christian life is not, is a bondage to the law, as if our salvation hung in the balance and depended on our meticulous and slavish obedience to the letter of the law. As it is, our salvation rests upon the finished work of Christ, on His sin-bearing, curse-bearing death, embraced by faith.”