14 Even if such men were slain in confession of the Name, that stain is not even washed away by blood: the inexpiable and grave fault of discord is not even purged by suffering. He cannot be a martyr who is not in the Church; he cannot attain unto the kingdom who forsakes that which shall reign there. Christ gave us peace; He bade us be in agreement, and of one mind. He charged the bonds of love and charity to be kept uncorrupted and inviolate; he cannot show himself a martyr who has not maintained brotherly love.
Paul the apostle teaches this, and testifies, saying, “And though I have faith, so that I can remove mountains, and have not charity, I am nothing. And though I give all my goods to feed the poor, and though I give my body to be burned, and have not charity, it profits me nothing. Charity is magnanimous; charity is kind; charity envies not; charity acts not vainly, is not puffed up, is not easily provoked, thinks no evil; loves all things, believes all things, hopes all things, endures all things Charity never falls.” “Charity,” says he, “never fails.”
For she will ever be in the kingdom, she will endure for ever in the unity of a brotherhood linked to herself. Discord cannot attain to the kingdom of heaven; to the rewards of Christ, who said, “This is my commandment that you love one another even as I have loved you:” he cannot attain who has violated the love of Christ b faithless dissension. He who has not charity has not God. The word of the blessed Apostle John is: “God,” says he, “is love; and he that dwells in love dwells in God, and God dwells in him.” They cannot dwell with God who would not be of one mind in God's Church.
Although they burn, given up to flames and fires, or lay down their lives, thrown to the wild beasts, that will not be the crown of faith, but the punishment of perfidy; nor will it be the glorious ending of religious valour, but the destruction of despair. Such a one may be slain; crowned he cannot be. He professes himself to be a Christian in such a way as the devil often feigns himself to be Christ, as the Lord Himself forewarns us, and says, “Many shall come in my name, saying, I am Christ, and shall deceive many.” As he is not Christ, although he deceives in respect of the name; so neither can he appear as a Christian who does not abide in the truth of His Gospel and of faith.
Source: The Treatises of Cyprian (New Advent)