The Eucharist is actually the one Christ, complete with His risen Body and Sacred Blood that's in living union with His immortal soul and divine personhood. It's really not a what but a Who.
This Blessed Sacrament is the same eternal Son of God who clothed Himself with humanity in Mary's womb 2,000 years ago, died and rose from the dead. The Eucharist is thus "the whole Christ," Who "is truly and substantially" present on the altar, teaches the 16th-century Council of Trent.
By Christ's words of consecration offered through the priest at Mass, the bread and wine become the substance of Christ's Body and Blood. The Catechism of the Catholic Church in paragraph 1376 refers to this radical transformation as transubstantiation.
The bread and wine don't actually change, however, into Christ's spiritual soul or divine nature. But all four parts are together. As St. Thomas Aquinas explains, this happens because wherever Christ's inseparable risen Body and Blood are present, there His soul and divinity are present through the principle of concomitance.
Thus, after the consecration, the bread changes into Christ's risen Body and the wine changes into Christ's Blood but the appearance of bread and wine remains. Eucharistic miracles have borne witness to this supernatural truth throughout the centuries and continue to do so.
But because Christ as the Blessed Sacrament remains veiled by the eucharistic appearances of bread and wine, various followers of Christ have struggled from the beginning to accept the mysterious fact of His Real Presence.
John 6:61 attests, "Many therefore of His disciples, hearing it, said: This saying is hard, and who can hear it?"
This supernatural mystery is thus rightly called "the mystery of faith."
The Eucharist is also called Holy Communion because in this way Catholics receive Christ in His risen Body. Reception of the smallest particle of what once was bread or the smallest drop of what once was wine is truly the reception of Christ — wholly and entirely.
See how Christ is truly among His flock in season one of Church Militant's Premium show, The One True Faith—Eucharist: The Real Presence.