So it is that the entire universal Church is rightly said to be the one and only bride of Christ, and each person, through the mystery of the Sacrament, is believed to be the Church in its fullness.
One in all and entire in each, holy Church is single in the plurality of its members thanks to the unity of faith, and manifold in each of them thanks to the bond of charity and the diversity of charisms, for they all come from One.
Although holy Church is thus diversified by the multiplicity of its members, it is nevertheless entirely fused into one by the fire of the Holy Spirit.
[...] This Spirit, therefore, is undoubtedly both one and multiple, one in majestic essence, multiple in diverse gifts; he fills the Church, and causes it to be both one in its universal extension, and whole in each of its parts.
[...] If those who believe in Christ are one, then through the mystery of the Sacrament the entire Body is present where bodily eyes see but a single member.
Solitude prevents no one from speaking in the plural; nor is it inappropriate for the multitude of believers to speak in the singular, for through the power of the Holy Spirit, who is present in each and fills all, it is clear that the solitude is full of people and the multitude forms a unity.
Our holy Fathers regarded this intimate relationship and communion of believers in Christ as so certain that they included it in the creed stating the Catholic faith, and commanded us frequently to call it to mind along with the other basic elements of Christian belief.
For immediately after we say: “I believe in the Holy Spirit, the holy Church” we add: “the communion of saints”.
Thus in the very act by which we bear witness to the God in whom we believe, we also affirm the communion that marks the Church which is one with him.
For this communion of saints in the unity of faith is such that, because they believe in one God, are reborn in one Baptism, and are strengthened by the one Holy Spirit, they are admitted, through the grace of adoption, into the one everlasting life.
Peter Damian (c.1007-1072): Liber Dominus Vobiscum, 5-6, 10 (PL 145:235-6, 239); from the Monastic Office of Vigils, Sunday of the Eighth Week of Ordinary Time, Year 1.