24 For even in the sacrifices which Abel and Cain first offered, God looked not at their gifts, but at their hearts, so that he was acceptable in his gift who was acceptable in his heart. Abel, peaceable and righteous in sacrificing in innocence to God, taught others also, when they bring their gift to the altar, thus to come with the fear of God, with a simple heart, with the law of righteousness, with the peace of concord. With reason did he, who was such in respect of God's sacrifice, become subsequently himself a sacrifice to God; so that he who first set forth martyrdom, and initiated the Lord's passion by the glory of his blood, had both the Lord's righteousness and His peace.
Finally, such are crowned by the Lord, such will be avenged with the Lord in the day of judgment; but the quarrelsome and disunited, and he who has not peace with his brethren, in accordance with what the blessed apostle and the Holy Scripture testifies, even if he have been slain for the name of Christ, shall not be able to escape the crime of fraternal dissension, because, as it is written, He who hates his brother is a murderer and no murderer attains to the kingdom of heaven, nor does he live with God. He cannot be with Christ, who had rather be an imitator of Judas than of Christ. How great is the sin which cannot even be washed away by a baptism of blood— how heinous the crime which cannot be expiated by martyrdom!
Source: The Treatises of Cyprian (New Advent)