Why was Cain's offering rejected?

Cain's offering was rejected because he approached God in self-reliance rather than through the lens of faith in God's commands. He brought the fruit of the ground, which represented the best of his own labor, but failed to recognize his need for a blood atonement. This self-righteousness, akin to the attitude of the Pharisees, signifies a critical misunderstanding of God's holiness and the necessity of substitutionary sacrifice. In contrast to Abel's heartfelt faith, Cain's worship was devoid of the humility that recognizes one's utter dependence on God's grace. Thus, God rejected Cain and his offering, illustrating that only those who come by faith, resting in Christ's righteousness, will be accepted.