Cain was upset at God because God had no respect for Cain's offering: through unbelief, Cain had offered a bloodless sacrifice. How did Cain know his sacrifice had not been accepted? We cannot be certain, but it is my opinion that God spoke to Cain by way of Cain's conscience: after Cain's sacrifice, there was no sense of forgiveness, but a reminder of his guilt. He could not be happy, for he yet bore his sin and knew it. Meanwhile, he saw Able full of the joy of the Lord and lighthearted for the burden of his sin had been lifted from his heart. This enraged Cain, as it does all religious but unsaved people. But, the Lord's admonition is as true today as it was in Cain's day, "If you do what is right, will you not be accepted?" There is no need for any man to be jealous of the blessings that believer's possess. There is no reason for unbelievers to rage at believers: any man alive may have what every believer has - peace with God. If anyone will come to God through Christ, confessing his sin and pleading the sacrifice of Jesus Christ, HE WILL BE ACCEPTED BY GOD. (Hebrews 7:25, 1 John 1:9, Hebrews 10:19). Let not the joy of believers be a stumbling block to you, but enter in to Christ as they have done, and find the same joy they have!