Genesis 4:5

but unto Cain and to his offering he had not respect. The Lord searches the reins and the heart. He noted the humble faith of Abel, whose one thought was to give the Lord a proof of the sincere gratitude for all the goodness and mercy which had been vouchsafed him. But God saw also the hypocrisy of Cain's heart, the fact that he was not interested in the worship which his hands were performing. He therefore indicated His pleasure in the one case and His displeasure in the other, either by some outward sign visible in the smoke of the offering, or by a subsequent rich blessing in the case of Abel, or through the mouth of Adam, as the priest of the family congregation. It is not the outward size of our gifts and offerings which makes them acceptable in the sight of the Lord, but the attitude of our hearts and minds toward God. He wants pure love flowing out of sound faith.

And Cain was very wroth, and his countenance fell. This paragraph shows the progress of actual sin, from the evil desire of the heart to the sinful act. Cain was jealous of his brother Abel because of the latter's humble faith and his consequent acceptance by God. He was angry exceedingly, he was filled with bitter wrath, which was reflected in his face, in the expression of his eyes, in his distended nostrils. He fell to dark brooding and evil plotting.