At this point in Scripture, Cain has killed his brother Abel in the first murder. God asks where Abel is and Cain doesn't tell God, however we know the God knew where Abel was. God sends judgment to Cain and makes a promise to Cain. The promise is basically that Cain will have a harder time working the ground. We know Cain's profession was a gardener so this had an incredible impact on his job. Cain exclaims that he could not bear that punishment, it was not true repentance but he was overwhelmed by the punishment.Cain says that people will seek him out and kill him for they would know he had killed Abel. God promises him that anyone who kills Cain will have the Lord's vengeance upon them. So Cain is safe.
It is an interesting promise to Cain, and one that does not affect us today because Cain is not with us, however there are a couple principles here. First, God will judge sin, God judges Cain for murdering his brother, and God will judge our sin. It may not be an immediate judgment, but it completely just and right. Second, even in judging Cain, there was a level of protection, God showed grace to Cain in that Cain would not be hunted down and killed. Believers are also protected by God, not physically, but we are secure in Him. Finally, there was a mark on Cain to distinguish him so nobody would harm him. God has given us the Holy Spirit and He works through us, we are to distinguish ourselves as belonging to God. We do not have a physical marking tha