Fr. St Cyprian—
God bids us to be peace-loving, harmonious and of one mind in his house; he wants us to live with the new life he gave us at our second birth. As sons of God, we are to abide in peace; as we have one Spirit, we should be one in mind and heart. Thus God does not receive the sacrifice of one who lives in conflict, and he orders us to turn back from the altar and be first reconciled with our brother, that God too may be appeased by the prayers of one who is at peace. The greatest offering we can make to God is our peace, harmony among fellow Christians, a people united with the unity of the Father, the Son and the Holy Spirit.
When Cain and Abel first offered their sacrifices, God considered not so much the gifts as the spirit of the giver: God was pleased with Abel’s offering because he was pleased with his spirit. Thus Abel the just man, the peacemaker, in his blameless sacrifice taught men that when they offer their gift at the altar they should approach as he did, in the fear of God, simplicity of heart, ruled by justice and peaceful harmony.
God cannot abide a murderer. He cannot be united with Christ, who has preferred to imitate Judas