I recognize that you need God.

"Then the Lord said to Cain: Why are you angry, and why is your countenance demolished? If you do good, will you not be accepted? And if you do not do good, sin lies at the door and covets you, but you you must master it. "

God bless my brothers of steemchucch.

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Why was Cain angry? He was angry enough to kill his brother. There is always anger behind a premeditated crime. The Lord said that if someone were angry with their brother for no reason, it is as if he were guilty of murder. Behind the anger there is jealousy, and behind jealousy, pride. And in spiritual pride there is no sense of sin. The Apostle James in his letter, in chapter 1:15, puts it this way:
"Later, when passion has conceived, it gives birth to sin, and when sin is consummated, it engenders death."
Cain's anger led to the crime, but behind this were his jealousy and pride.

And God dealt with him in the following way. He said: if you do well, will you not be accepted? The question would be better translated by the expression "will not you achieve excellence?" The eldest son always occupied a place of preeminence and this young man must have thought that he would lose that privilege. God told him that if he acted correctly, there was no reason for him to lose it. And to act well would have been to present before God what he had accepted from Abel, that is, a sacrifice and the recognition that he was a sinner. But Cain continued to be dominated by anger.

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Some have interpreted it as meaning that an offering for sin lay at the door. That is, a small lamb was ready at the door. In a sense, it could be true, but I do not believe that in this passage there is a reference to the sin offering. Until this moment and later, actually until the time of Moses, as far as I can deduce from the Holy Scriptures, there was not yet the sin offering. The instructions regarding the sin offering are found in the book of Leviticus, in the first part of which there are 5 types of offerings, which we will opportunely study, one of which is the sin offering. This offering did not arise until the delivery of the law to Moses. In his letter to the Romans, chapter 3:20, the apostle Paul says that "... through the law comes the knowledge of sin." The offerings that were offered until that time and, specifically in the time that concerns us, were the so-called "offerings on fire". Abel, and later Noah, Abraham and Job, offered these offerings. In them, the animal was totally consumed by fire on the altar. They prefigured Christ in his sacrifice on the cross occupying our place, as an offering to God that He accepts for our sins.

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It is obvious that Cain did not realize how vulnerable he was to sin. When God told him that sin lay at the door, I think he gave him to understand that sin was like a wild beast crouched to pounce on him as soon as he entered the entrance. For this reason, Cain needed to offer a sacrifice for sin that would be acceptable to God, a sacrifice that would point to Christ.