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The holy universal Church is one Body, constituted under Jesus Christ its Head … Therefore, Christ, with His whole Church, both that which is still on earth and that which now reigns with Him in Heaven, is one person … Therefore, the Apostle says: “from Whom the whole body being compacted and fitly joined together makes increase of the body” (Eph. 4:16). This is that Body outside which the Spirit does not give life; wherefore, St. Augustine says: “If you would live by the Spirit of Christ, be in the Body of Christ.” Of this Spirit the heretic does not live, nor the schismatic, nor the excommunicated, for they are not of the Body.
– Pope St. Gregory the Great