Why and how is the Church one?
The Church is one because there is one Head, Jesus Christ, and therefore one Body, animated by the one Spirit of God. As the Scriptures teach: “There is one body and one Spirit, even as you also were called to one hope when you were called; one Lord, one faith, one baptism, one God and Father of all” (Eph 4:4–6; see Q283, Q303); “As the body is one and yet has many members, and all the members of the body (though many) form one body, so also is Christ. For in one Spirit we were all baptized into one body, both Jews and Greek, slaves or free; and we were all made to drink into one Spirit” (1Co 12:12–14; see Q260).
Oneness is a fundamental property of the Church, and this oneness can never be torn asunder, like the unity of the Eucharist which constitutes her as the Body of Christ. The Divine Liturgy proclaims at the fraction:
Broken and distributed is the Lamb of God: broken, but not divided; forever eaten, yet never consumed; sanctifying those who partake! (Divine Liturgy of St John Chrysostom, Rite of Fraction)
The oneness of the Church applies both to her transcendent aspect, the Body of Christ containing all who have been united to the Savior in the course of this age, and to her manifestations in space and time. This oneness is revealed by the one Head, Christ Jesus, and, on earth, by the one bishop in each church, who is head in a representative and derivative sense, without prejudice to the divine headship of Christ.
The oneness of the Church is intimately connected with her attribute of being “catholic”: it is catholicity, wholeness and fullness in each place, that ensures the oneness of the Church in both her transcendent and her localized expressions.
