The Doctrine of Concomitance
The doctrine of concomitance states that the whole Christ, body, blood, soul, and divinity, is contained under both species (bread and wine) in the Eucharist. This is why the Latin custom of withholding the chalice from the laity is not, in fact, a violation of the Savior’s commandments. The command to “drink of [the cup], all of you,” was not given to all of the Lord’s disciples, but rather to the Apostles alone (Matt 26:20, 27). We know this because it was joined to the command to “do this in remembrance of me” (Lk 22:19), i.e. consecrate and offer the Eucharist, something only the Apostles and their successors can do. This is the reason why all priests must drink of the chalice in order to validly celebrate Mass, with no exceptions whatsoever, whereas the laity are not bound by this precept. This is also why few people question the practice of not handing the bread and wine directly to all communicants, instead being fine with just the priests ministering the sacrament. However, the command that was given to all of Christ’s followers was to eat His body and drink His blood (Jn 6:54). Thanks to the doctrine of concomitance, the faithful can do this simply by partaking of one species of the Eucharist (either the bread or the wine).
This doctrine has two foundations, one from reason and the other from Scripture. From reason, we know that Christ’s deified human nature cannot be broken up into separable parts, and so every particle of the Eucharist must contain the whole Christ. From Scripture, 1 Corinthians 11:27 is the clearest articulation of concomitance: “Whoever, therefore, eats the bread or (ἢ) drinks the cup of the Lord in an unworthy manner will be guilty concerning the body and (καὶ) blood of the Lord.” According to St. Paul, if you either partake of the species of bread or the species of wine unworthily, you will be guilty concerning both the body and the blood of Christ. This teaching only makes sense if the whole Christ, body, blood, soul, and divinity, is contained under both Eucharistic species (it also may point to some communicants only receiving one species in apostolic times). Thus, the Savior could truly say, with no allusion to the chalice, “If anyone eats of this bread, he will live forever. And the bread that I will give for the life of the world is my flesh” (Jn 6:51).