"Because Catholics believe that the celebration of the Eucharist is a sign of the reality of the oneness of faith, life, and worship, members of those churches with whom we are not yet fully united are ordinarily not admitted to Communion. Eucharistic sharing in exceptional circumstances by other Christians requires permission according to the directives of the diocesan bishop and the provisions of canon law. . . . "
Scripture is clear that partaking of the Eucharist is among the highest signs of Christian unity: "Because there is one bread, we who are many are one body, for we all partake of the one bread" (1 Cor. 10:17). For this reason, it is normally impossible for non-Catholic Christians to receive Holy Communion, for to do so would be to proclaim a unity to exist that, regrettably, does not.
Another reason that many non-Catholics may not ordinarily receive Communion is for their own protection, since many reject the doctrine of the Real Presence of Christ in the Eucharist. Scripture warns that it is very dangerous for one not believing in the Real Presence to receive Communion: "For any one who eats and drinks without discerning the body eats and drinks judgment upon himself. That is why many of you are weak and ill, and some have died" (1 Cor. 11:29–30).