Moreover, communion for Protestants is not what it is for Catholics. We believe that Jesus Christ is truly present in the Holy Eucharist (Body, Blood, Soul, and Divinity); He really is present because bishops who enjoy an unbroken continuity with the original apostles, ordained by Christ, have validly ordained our priests. Protestants, on the other hand, do not have the Real Presence, nor do most of them even believe in the Real Presence in Communion. The only time that a Catholic may licitly receive the Eucharist in a non-Catholic Church is spelled out clearly in Canon 844.2:
"Whenever necessity requires or a genuine spiritual advantage commends it, and provided the danger of error or indifferentism is avoided, Christ's faithful for whom it is physically or morally impossible to approach a Catholic minister, may lawfully receive the sacraments of penance, the Eucharist and anointing of the sick from non-Catholic ministers in whose Churches these sacraments are valid (emphasis added)."
It is commonly understood that these sacraments are valid in the Eastern Orthodox churches, as well as the Assyrian Church of the East and the Polish National Catholic Church. But they are not considered valid in the Protestant churches. - FH
Sample extract from Catholic Answers to Catholic Questions (page 87).
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