"Then the LORD said to Cain, 'Where is Abel your brother?' He said, 'I do not know. Am I my brother's keeper?' And He said, 'What have you done? The voice of your brother's blood cries out to Me from the ground. So now you are cursed from the earth, which has opened it mouth to receive your brother's blood from your hand. When you till the ground, it shall no longer yield its strength to you. A fugitive and a vagabond you shall be on the earth.' And Cain said to the LORD, 'My punishment is greater than I can bear! Surely You have driven me out this day from the face of the ground; I shall be hidden from Your face; I shall be a fugitive and a vagabond on the earth, and it will happen that anyone who finds me will kill me.' And the LORD said to him, 'Therefore, whoever kills Cain, vengeance shall be taken on him sevenfold.' And the LORD set a mark on Cain, lest anyone finding him should kill him" (Gen. 4:9-15).
After Adam and his wife sinned in the garden, God asked questions initially, allowing them a chance to confess. He does likewise here with Cain. Although Adam made excuses for his sin, he did not lie. Cain both lies to God and manifests a hardened heart void of compassion. He shows no remorse for his sin! God then rebukes him and punishes him severely, though due to His longsuffering nature does not take his life. After Adam's sin, all humans would have greater difficulty with the soil, but Cain's struggle with the earth would be excessive. Cain's crime would make him an outcast among other humans. He feared that someone would kill him. God did not desire this and placed some sort of mark on Cain. Some, with no evidence whatsoever, have suggested that the marking of Cain explains the origin of dark-skinned people. Friends, there is no proof of this. In fact, one could just as well argue that the mark of Cain was an extreme lightening of his skin tone (to explain the origin of light-skinned people). Genetically, it makes most sense to believe that Adam and Eve were both brown skinned and had the genetic potential to produce both light and dark-skinned children. Thus, although we cannot know what precisely the mark of Cain was, it seems foolish to affirm that it had anything to do with melanin content in the skin. It is so tragic to see Cain express no sorrow for his sin; he is only concerned about himself and grieves over the punishment God inflicted (cf. II Cor. 7:9,10). We have seen two examples now already of the truthfulness of Proverbs 13:15 - "The way of the unfaithful is hard."
"Then Cain went out from the presence of the LORD and dwelt in the land of Nod on the east of Eden. And Cain knew his wife, and she conceived and bore Enoch. And he built a city, and called the name of the city after the name of his son--Enoch" (Gen. 4:16,17). Cain found a sister (presumably) to marry him (and he may have married prior to slaying his brother). Cain is able to have a son and build a city. Now, don't make the mistake here of defining a "city" by our modern terms. It was likely nothing more than a couple tents perhaps with some sort of fence for protection. The history of Cain's descendants is not central to Moses' narrative, so it moves very quickly. The seventh generation from Adam (via Cain) is Lamech, an exceedingly wicked man. He seems to have been the originator of polygamy, as 4:19 records him taking two wives. Even worse, he brags about killing a man! "Then Lamech said to his wives: 'Adah and Zillah, hear my voice; wives of Lamech, listen to my speech! For I have killed a man for wounding me, even a young man for hurting me. If Cain shall be avenged sevenfold, then Lamech seventy-sevenfold'" (Gen. 4:23,24). Lamech is here stating his planned course of behavior towards any who would wrong him in the slightest--he would punish them severely! Here is a man full of the pride of life, the lust of the flesh, and the lust of the eyes! It is important to note in this section of text that certain trades and skills are already being mentioned (e.g., raising livestock [perhaps men became meat eaters here in violation of 1:29], playing harps and flutes, craftsman in both bronze and iron). Obviously, early mankind was full of intellect and ingenuity and attempted to improve his comfort on Earth. He quickly advanced in many skills, aided by extreme initial longevity, of course. Think of all the things (good or bad) that one could learn, discover, and master in 900 plus years of life!
"And Adam knew his wife again, and she bore a son and named him Seth, 'For God has appointed another seed for me instead of Abel, whom Cain killed.' And as for Seth, to him also a son was born; and he named him Enosh. Then men began to call on the name of the LORD" (Gen. 4:25,26). This is certainly not in order chronologically (cf. 5:3). Seth would have been born long before Lamech. Why is this third son mentioned but none of the others? Because the history of the world would unfold through Seth (via Noah, who is introduced at the end of chapter 5). After Seth was born, it appears that humans began engaging in some sort of public worship. Private worship had already been practiced previously. Although God spoke initially to Adam, Eve, and their children, this practice did not last long. So far as speaking directly to man is concerned, God became mostly silent from this point on, making exceptions for certain patriarchs and prophets. After only a few generations passed, there would be some perhaps who began questioning the existence of the LORD. This doubt would lead to all kinds of immorality (which is made quite clear in Gen. 6).