6 The spouse of Christ cannot be adulterous; she is uncorrupted and pure. She knows one home; she guards with chaste modesty the sanctity of one couch. She keeps us for God. She appoints the sons whom she has born for the kingdom. Whoever is separated from the Church and is joined to an adulteress, is separated from the promises of the Church; nor can he who forsakes the Church of Christ attain to the rewards of Christ. He is a stranger; he is profane; he is an enemy.
He can no longer have God for his Father, who has not the Church for his mother. If any one could escape who was outside the ark of Noah, then he also may escape who shall be outside of the Church. The Lord warns, saying, “He who is not with me is against me, and he who gathers not with me scatters.” He who breaks the peace and the concord of Christ, does so in opposition to Christ; he who gathers elsewhere than in the Church, scatters the Church of Christ. The Lord says, “I and the Father are one;” and again it is written of the Father, and of the Son, and of the Holy Spirit, “And these three are one.” And does any one believe that this unity which thus comes from the divine strength and coheres in celestial sacraments, can be divided in the Church, and can be separated by the parting asunder of opposing wills?
He who does not hold this unity does not hold God's law, does not hold the faith of the Father and the Son, does not hold life and salvation.
Source: The Treatises of Cyprian (New Advent)