In light of this and other scriptural passages, the Church obliges Catholics to make sure they are properly disposed to receive the Eucharist before approaching the altar. For example, they must not receive Holy Communion in a state of mortal sin (see “Does the Bible Distinguish Between Mortal and Venial Sins?”). But there are other dispositions necessary as well. Those who deny that the Eucharist is truly the Body and Blood of Christ, believing it to be just a symbol, should also refrain from receiving Communion. To use the apostle’s words, they would be eating and drinking “without discerning the body” properly, placing themselves in danger of judgment.
This is the reason why non-Catholics, Protestants in particular, should not partake in the Eucharist in a Catholic church: They typically deny that it is truly Christ’s Body and Blood. But the Church insists that even Protestants who believe what the Catholic Church teaches about the Eucharist should not normally approach the Catholic altar for Communion. The reason for this restriction is suggested by other words from St. Paul:
The cup of blessing that we bless, is it not a participation [or fellowship] in the blood of Christ? The bread that we break, is it not a participation [fellowship] in the body of Christ? Because the loaf of bread is one, we, though many, are one body, for we all partake of the one loaf. [1 Cor 10:16-17].
The Eucharist thus signifies a oneness in faith, life, and worship among those who partake of it. So reception of the Eucharist by those separated from the Catholic Church is in a sense dishonest. It implies a unity that does not yet exist. (A few exceptions are allowed for pastoral reasons in extraordinary situations; see the Catechism 1401.)
In the meantime, the Church urges us to pray fervently that all Christians might finally “attain to the unity of faith and knowledge of the Son of God” (Eph 4:13).
RELATED SCRIPTURE — Texts cited: 1 Cor 10:16-17; 11:27-29; Eph 4:13. General: Ps 133:1-3; Jn 18:20-23; 1 Cor 1:10; Eph 4:1-6; Phil 2:1-2. CATECHISM OF THE CATHOLIC CHURCH — 805; 838; 1396; 1398-1401