But unto Cain and to his offering he had not respect. And Cain was very wroth, and his countenance fell.

Wesley's Notes

4:5 And Cain was wroth, and his countenance fell - Not so much out of grief as malice and rage. His sullen churlish countenance, and down - look, betrayed his passionate resentment.

Matthew Henry's Concise Commentary

4:1-7 When Cain was born, Eve said, I have gotten a man from the Lord. Perhaps she thought that this was the promised seed. If so, she was wofully disappointed. Abel signifies vanity: when she thought she had the promised seed in Cain, whose name signifies possession, she was so taken up with him that another son was as vanity to her. Observe, each son had a calling. It is the will of God for every one to have something to do in this world. Parents ought to bring up their children to work. Give them a Bible and a calling, said good Mr. Dod, and God be with them. We may believe that God commanded Adam, after the fall, to shed the blood of innocent animals, and after their death to burn part or the whole of their bodies by fire. Thus that punishment which sinners deserve, even the death of the body, and the wrath of God, of which fire is a well-known emblem, and also the sufferings of Christ, were prefigured. Observe that the religious worship of God is no new invention. It was from the beginning; it is the good old way, Jer 6:16. The offerings of Cain and Abel were different. Cain showed a proud, unbelieving heart. Therefore he and his offering were rejected. Abel came as a sinner, and according to God's appointment, by his sacrifice expressing humility, sincerity, and believing obedience. Thus, seeking the benefit of the new covenant of mercy, through the promised Seed, his sacrifice had a token that God accepted it. Abel offered in faith, and Cain did not, Heb 11:4. In all ages there have been two sorts of worshippers, such as Cain and Abel; namely, proud, hardened despisers of the gospel method of salvation, who attempt to please God in ways of their own devising; and humble believers, who draw near to him in the way he has revealed. Cain indulged malignant anger against Abel. He harboured an evil spirit of discontent and rebellion against God. God notices all our sinful passions and discontents. There is not an angry, envious, or fretful look, that escapes his observing eye. The Lord reasoned with this rebellious man; if he came in the right way, he should be accepted. Some understand this as an intimation of mercy. If thou doest not well, sin, that is, the sin-offering, lies at the door, and thou mayest take the benefit of it. The same word signifies sin, and a sacrifice for sin. Though thou hast not done well, yet do not despair; the remedy is at hand. Christ, the great sin-offering, is said to stand at the door, Re 3:20. And those well deserve to perish in their sins, that will not go to the door to ask for the benefit of this sin-offering. God's acceptance of Abel's offering did not change the birthright, and make it his; why then should Cain be so angry? Sinful heats and disquiets vanish before a strict and fair inquiry into the cause.