ὅτι εἷς ἄρτος, ἓν σῶμα οἱ πολλοί ἐσμεν, οἱ γὰρ πάντες ἐκ τοῦ ἑνὸς ἄρτου μετέχομεν. (1 Cor 10:17)
"Since there is one Bread, we who are many are one Body; for we all partake of the one Bread." (1 Cor 10:17)
In my recent article "The Sacrilege of Schism" I discussed the three bonds of unity. One of those bonds is "common celebration of divine worship, especially of the sacraments" (CCC 815). In order to achieve the full and visible unity of all Christ's followers, we must share the same sacraments. This is not only because we need to "agree" and "be made complete in the same mind and in the same judgment". (1 Cor 1:10) Sacramental unity is not reducible to formal or doctrinal unity. Agreement in doctrine is one way we are made one, but merely agreeing on doctrine is not sufficient to make us one. The sacraments themselves actually operate to make us one Body. That is why I discussed here the way in which the sacrament of baptism makes us one, and then more recently I discussed here the way the sacrament of confirmation makes us one. The third sacrament among the sacraments of initiation is the sacrament of the Eucharist. How does the Eucharist make us one?
As St. Paul explains in 1 Corinthians 10:17, it is because we all partake of the one Bread that we are all one Body. This is the Bread that came down from heaven, of which the manna that the Hebrews ate was but a type. This Bread is greater than the manna. Those who ate the manna died. But those who eat this Bread live forever. (John 6)
This is what we need to understand about the Eucharist and unity. When we eat regular bread, we digest it and it ceases to exist as bread, and becomes incorporated into our body. But when we eat the Body of Christ and drink His Blood, it works the other way around; we actually become a part of Him. That is because when two things become one, the entity with the greater unity incorporates the entity with the lesser unity into itself. This is why when the Logos became man, human nature was elevated to divinity. If you were to touch the skin of baby Jesus in the lap of Mary, you were literally touching God. That is why the Third Ecumenical Council (Ephesus 431) used the term "Theotokos" as a test of orthodoxy against the Nestorians. The term "Theotokos" wasn't so much about Mary, per se, as about the deity of the baby that came from her womb. The baby that came from her womb was God. Since she was the mother of the baby that came from her womb, she was the mother of God, or the God-bearer (Theotokos). The point here is that in the hypostatic union, the human nature of Christ, without ceasing to be human, became divine, was taken up into and became one with the Logos. That is why when we eat the Body and Blood of Christ, who is perfect unity, we are taken up into and become one with Him. If we understand that a greater unity incorporates a lesser unity into itself, and that Christ has greater unity than do we, then we can better understand how partaking of His Body makes us all one. We are one Body, because we all partake of one Bread. Those who do not partake of the one Bread, [ordinarily] cannot be fully united with the one Body.
What do the fathers think about the Eucharist? I collected some quotations from the fathers regarding the Eucharist here.
Neil Babcox was a Presbyterian pastor for many years, and recently became a Catholic. One of the main reasons he became a Catholic had to do with the Eucharist. He describes in this video the role the Eucharist played in bringing him into the Catholic Church. A further description of Neil's reception into full communion with the Catholic Church is described here.
"Let unity, the greatest good of all goods, be your preoccupation." - St. Ignatius of Antioch (Letter to St. Polycarp)