Question
Can I go to communion with my neighbor some times in his Methodist church? I'm a Catholic. What about they receiving at my church some times? They believe it's the Body and Blood of Christ too.
--Similar in Scranton
Answer
From our noted ecumenist Rev. Ernest Falardeau, SSS:
It is difficult to reply in a few words to the question: "May Protestants and Other Christians receive in the Catholic Church?" Even more difficult is the question: "May Catholics receive in other Christian Churches?"
The answer can be found in the Code of Canon Law (can. 844) and in the Directory for the Application of Principles and Norms for Ecumenism (published in 1993).
These sources state simply that normally Christians should receive in their own Churches. However it is possible for Christians to share the Eucharist across Church lines under certain circumstances by way of exception (persons must be baptized, they must be unable to receive from their own minister, they must have the same understanding of the Eucharist as the Catholic Church, they must receive for spiritual benefit, they must approach the Eucharist on their own accord.)
A "rule of thumb" that might be useful for further discernment is that the degree of communion that exists between the Churches concerned determines the degree of possibility for eucharistic sharing. Also important, according to Catholic teaching, is the valid ordination of ministers.
Thus, because Orthodox and Ancient Eastern Churches are considered to be in "almost full communion" and because their ordinations have always been recognized as valid by the Catholic Church, according to canon 844 Catholics may receive in the Orthodox Church and Orthodox may receive in the Catholic Church. Indeed the East Syrian and West Syrian (Ancient Eastern) Churches explicitly allow such eucharistic sharing. The same is true of the Old Catholic and Polish National Churches which are considered to be in the same condition as the Orthodox Churches (cf. canon 844#3).
As for other Christian Churches, reciprocity does not exist because the Roman Catholic Church has not recognized the validity of their ordinations. Thus Catholics may not, according to existing legislation, receive in those Churches. However Protestants may receive in the Catholic Church when they fulfill the conditions indicated in the documents cited above.
For further reference see:
Ernest Falardeau "Sacramental Sharing: A Theological Perspective from the New Ecumenical Directory". Ecumenical Trends 24 (1995) 113-114, 121-124.
Ladislas Orsy. "Interchurch Marriages and the Reception of the Eucharist" America 175(1996) 18-19, [Oct. 12, 1996, no. 10]