I think the title sums it up. I'm a protestant Christian, and have many Catholic friends. When I visit their churches, is it permissible for me to participate in the Eucharist/Communion ceremony by partaking of the bread and wine?
The answer is: "more than likely not".
Catholics do allow others to take part in the Holy Communion, but they maintain tight restrictions on this.
Guidelines for the Reception of Communion
For Other Christians
... 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 Holy Communion.
...Members of the Orthodox Churches, the Assyrian Church of the East, and the Polish National Catholic Church are urged to respect the discipline of their own Churches. According to Roman Catholic discipline, the Code of Canon Law does not object to the reception of communion by Christians of these Churches.
...While we cannot admit them to Holy Communion, we ask them to offer their prayers for the peace and the unity of the human family.
So, when they say "not fully united", Protestants generally fall into this category. (I'm not sure where those other churches fall in the "Protestant-Catholic" spectrum, so I'm leaving this a bit generic.) So, therefore, Protestants are not admitted to Holy Communion.
The only safe thing to do is to ask the priest beforehand. To willingly participate in someone else's communion when they would say "no" if they knew your story is offensive. As a result, the only option I see is to ask them.
If you explain your position, and that you are [insert denomination here], but visiting their congregation with your friends, is it OK if I participate in Communion. The biggest sticking point is likely to be transubstantiation, and it's a biggie.
If they say no, they will most likely still permit you to receive a blessing, which everyone is entitled to.
Sorry, I don't mean to say that you have offended them, but that is a risk you run when you don't ask
tldr version: It is not normally permissible by canon law for you to participate in the Eucharist. However, you can still join the communion line and indicate you would like a blessing by placing your index finger over your lips (similar to shhing someone) or by crossing your arms over your chest in the shape of an 'X' when you approach the minister.
The Canon law regarding reception of Holy Communion is given below.
§1 Catholic ministers may lawfully administer the sacraments only to catholic members of Christ's faithful, who equally may lawfully receive them only from catholic ministers, except as provided in §§2, 3 and 4 of this canon
§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.
§3 Catholic ministers may lawfully administer the sacraments of penance, the Eucharist and anointing of the sick to members of the eastern Churches not in full communion with the catholic Church, if they spontaneously ask for them and are properly disposed. The same applies to members of other Churches which the Apostolic See judges to be in the same position as the aforesaid eastern Churches so far as the sacraments are concerned.
§4 If there is a danger of death or if, in the judgement of the diocesan Bishop or of the Episcopal Conference, there is some other grave and pressing need, catholic ministers may lawfully administer these same sacraments to other christians not in full communion with the catholic Church, who cannot approach a minister of their own community and who spontaneously ask for them, provided that they demonstrate the catholic faith in respect of these sacraments and are properly disposed.
§1 States that only Catholics may receive communion, with a few exceptions.
§2 deals with Catholics receiving sacraments from a non-catholic minister.
§3 Allows members of certain specifically named Churches to receive.
§4 Allows any non-Catholic Christian to receive Holy Communion under extraordinary circumstances, "provided that they demonstrate the catholic faith in respect of these sacraments". In respect to Holy Communion, the Catholic faith is that of transubstantiation, that is, the substance of the bread and wine is changed into the body, blood, soul, and divinity of Jesus Christ.
Thus, it appears that Christians who do not hold the belief of transubstantiation as the Catholic Faith teaches are not admitted to Holy Communion.
This, however, does not mean you cannot join in the worship and community of the Mass. At communion time, you can participate in the community by requesting a blessing rather than reception of the Eucharist. This is done either by placing your index finger over your lips or by crossing your arms over your chest when you approach the Eucharistic Minister.
According to Catholic teachings on this topic? Generally no (as others have referenced).
Would a particular priest be stupid and allow it? Maybe - some didn't have the best priestly formation.
Would it be good for you spiritually? No.
The most compassionate explanation I've heard is this:
When you go up to receive communion the priest says "Body of Christ" and you reply with "Amen". This "AMEN" is an agreement that what the Catholic Church teaches about transubstantiation is TRUE and that the Church has the AUTHORITY to DO this. You are agreeing that the Catholic Church has the authority to make Christ physically present - body, soul, and divinity - in the blessed sacrament.
- If you do believe this - why aren't you Catholic?
- If you don't believe this then the Church is either crazy or idolatrous. Why do you want to receive something that so fundamentally conflicts with what you hold to be true?
To say "yes I believe" when you in fact don't, is a lie. To speak a lie like this, about such a crucial topic, will tear at you spiritually and can damage your relationship with the Lord. Out of concern for your spiritual well-being the Church asks non-Catholics to refrain from receiving:
For those who eat and drink without discerning the body of Christ eat and drink judgment on themselves. That is why many among you are weak and sick, and a number of you have fallen asleep. (1 Corr 11)
The answer is basically “no,” there is no Protestant denomination whose members would be admitted to Communion.
The Church admits to Communion Catholics (evidently) and all those Christians who belong to a fully apostolic Church: that is, to a Church that has maintained apostolic succession and has the Catholic Faith regarding the Eucharist. This includes all Orthodox Churches, the Oriental Orthodox, the Armenian Orthodox, and the Assyrian Church of the East. (There may be some smaller chuches that I am omitting.)
(For the benefit of readers, the Catholic Church, and all the churches mentioned above, teach that in the celebration of the Eucharist, the bread and wine are fully converted into Jesus Christ. Once the Consecration has taken place, no bread or wine remain, but only Christ. Or else, in more technical terms, the substance of the bread and wine is changed to the substance of Jesus. The appearances, or accidents, remain those of bread of wine. This doctrine, which is based on the institution narratives in the Bible—that is, Mt. 26:26-29, Mk. 14:22-25, Lk 22:19-20, as well as John 6 and St. Paul’s instructions on the Lord’s supper in 1 Corinthians—was given the technical term transubstantiation in the late Middle Ages.)
The reason that Protestants are not admitted to Communion in the Catholic Church is that the vast majority do not accept this understanding of the Eucharist. Moreover, the ministers in Protestant churches do not have the ability to confect the Eucharist; indeed the vast majority of Protestants do not accept the concept of apostolic succession (and those denominations that have preserved some aspect of it—such as the Anglicans—in fact, lost their apostolic succession at the time of the Reformation; see Apostolicae curae by Pope Leo XIII).
The Code of Canon Law Can. 844 goes into the specifics, including certain exceptional circumstances under which Protestants might receive the Eucharist:
§1. Catholic ministers administer the sacraments licitly to Catholic members of the Christian faithful alone, who likewise receive them licitly from Catholic ministers alone, without prejudice to the prescripts of §§2, 3, and 4 of this canon, and can. 861, §2.
Hence, in general, only Catholics should receive Communion from the Catholic Church.
Canon Law says, regarding non-Catholics receiving Communion:
§3. Catholic ministers administer the sacraments of penance, Eucharist, and anointing of the sick licitly to members of Eastern Churches which do not have full communion with the Catholic Church if they seek such on their own accord and are properly disposed. This is also valid for members of other Churches which in the judgment of the Apostolic See are in the same condition in regard to the sacraments as these Eastern Churches.
Members of Eastern Churches (and any church in a similar circumstance) may approach Communion in a Catholic Church; however, they are urged to follow the disciplines of their own churches. (See below.)
§4. If the danger of death is present or if, in the judgment of the diocesan bishop or conference of bishops, some other grave necessity urges it, Catholic ministers administer these same sacraments licitly also to other Christians not having full communion with the Catholic Church, who cannot approach a minister of their own community and who seek such on their own accord, provided that they manifest Catholic faith in respect to these sacraments and are properly disposed.
Hence, Protestants could receive Communion when in danger of death or in some other grave necessity, provided they professed the Catholic faith.
The United States Conference of Catholic Bishops in 1996 issued a set of guidelines for receiving communion that summarizes the Church’s position very well. The relevant passage says,
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 Holy 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 (canon 844 §4). Members of the Orthodox Churches, the Assyrian Church of the East, and the Polish National Catholic Church are urged to respect the discipline of their own Churches. According to Roman Catholic discipline, the Code of Canon Law does not object to the reception of Communion by Christians of these Churches (canon 844 §3).
No, it is not permissible for one who does not believe in the Real Presence of Christ in the Eucharist to receive Communion. You surely do not, or you wouldn't refer to is as the "bread and wine".
This article at Catholic Answers is very clear.
You ask if you "may". To answer that, it is necessary to check first what authority are you seeking to "allow" it to you. Do you care about offending other Catholics in some particular church assembly? About crossing the Catholic rules or doctrine? Do you want to know if a particular priest administering the communion will have any objections?
- most protestant churches have nothing against their members participating in Catholic communion
- from the Catholic side there are some conditions
- if you are after official Catholic church doctrine, you need to follow the following rules (see also Who Can Receive Communion?):
- you need to believe the bread and wine are becoming the body and blood of Christ (transubstantiation)
- you should be allowed by a bishop (CCC 1401, CIC 844 §4)
- you need to follow the same rules as Catholics do, i.e:
- do not eat or drink anything else but water one hour before the communion
- you need to go to confession before communion (if you're in Mortal Sin).
- in practice, individual Catholics including priests apply much more liberal rules, sometimes they even actively encourage protestants to participate in the communion. If you are interested about this, you need to ask the priest, or the people visiting the church if the priests attitude is known to them.
- if you are after official Catholic church doctrine, you need to follow the following rules (see also Who Can Receive Communion?):
I have seen some Catholics be very offended by Protestants taking the Eucharist. However, I also know many confessing Catholics who have no idea what transubstantiation is about and hold quite a Protestant view about Communion.
Being a Protestant raised in a Catholic family, this is a question I've asked myself. In general, I avoid taking it, but that can sometimes raise discussions if I don't take it like everyone else does during Christmas mass when visiting family for holidays.
I think Farseeker's point of asking the priest is a good one, when it is possible.
I once was present at a Catholic mass, and the priest, knowing that a large number of protestants (Calvinists) attended the event, explicitly stated that the protestant brothers* can also participate if they wish to do so.
So, the event and the historical, cultural etc. background can have an influence. Usually it's safer to ask, especially if you are foreign to the community and don't know their customs.
* note that the language the mass was held in is genderless, so "brothers" was gender-neutral
I don't think you should. Here's what the reformers thought (quoting from the Heidelberg Catechism):
Q80. What difference is there between the Lord's supper and the papal mass? A. The Lord's supper testifies to us, first, that we have complete forgiveness of all our sins through the one sacrifice of Jesus Christ, which he himself accomplished on the cross once for all;  and, second, that through the Holy Spirit we are grafted into Christ,  who with his true body is now in heaven at the right hand of the Father,  and this is where he wants to be worshipped.  But the mass teaches, first, that the living and the dead do not have forgiveness of sins through the suffering of Christ unless he is still offered for them daily by the priests; and, second, that Christ is bodily present in the form of bread and wine, and there is to be worshipped. Therefore the mass is basically nothing but a denial of the one sacrifice and suffering of Jesus Christ, and an accursed idolatry.
 Mt 26:28; Jn 19:30; Heb 7:27; 9:12, 25, 26; 10:10-18.  1 Cor 6:17; 10:16, 17.  Jn 20:17; Acts 7:55, 56; Heb 1:3; 8:1.  Jn 4:21-24; Phil 3:20; Col 3:1; 1 Thess 1:10.
The Eucharist is the Body and Blood of Christ. If you think it is just wine and bread, that almost certainly means no, you cannot receive it.
Basic requirements to receive Communion include:
Fasting for one hour prior (This actually used to be all day.)
Not having any unrepented grave sin
Belief in the Transubstantiation.
Of course, sometimes Protestants fulfill these requirements and Catholics don't. (There was a point where I, a cradle Catholic, didn't. I also know a convert from Protestantism who had a hard time waiting until Confirmation to receive the Eucharist when she firmly believed in the Transubstantiation.) The presence of bad Catholics does not nullify the teachings.
This is one of those cases where it's much better to be safe than sorry. Unless you ask a priest personally and he allows it, refrain from receiving the Eucharist.
There are two sides to this issue. The first is whether the Catholics consider it appropriate to share communion with those who understand it differently. As a Christian, I want to avoid offending any fellow believer, so I think it would be important to ask if in doubt.
The second point is your own conscience. If you feel that the ceremony or understanding of the Roman Catholics is inappropriate to your relationship with Christ, then you might want to avoid partaking even if they allow you to.
Communion is meant to be a shared experience and an affirmation of your relationship to Christ. I believe nearly all denominations that practice it regard it as an important moment, not to be done lightly or in a wrong spirit. On the other hand, I would seek the guidance of the Holy Spirit if unsure. I know of one occasion where a Roman Catholic priest, in a mixed-denomination situation, invited non-Catholics to partake. In that case, I might ask the Holy Spirit and partake if I felt no twinge of conscience.
The case is not only, or even mainly, that it is not permissible, but that it is definitely not advisable for your own good. Because if you received Communion in a Church which has maintained apostolic succession, without yourself believing that after consecration there is no longer any bread or wine but the Body and Blood of our Lord Jesus Christ, your case would be exactly the one described by Paul in this passage:
For anyone who eats and drinks without discerning the Body eats and drinks judgment on himself. That is why many of you are weak and ill, and some have died. (1 Cor 11:29-30. ESV.)
In all Apostolic Churches (Catholic, Eastern Orthodox, Oriental Orthodox which includes Armenian and, from the viewpoint of the Catholic Church only I think, the Assyrian Church of the East), the bishop or presbyter celebrating the Divine Liturgy (commonly called "Mass" by Catholics) has apostolic succession and therefore effects the real change of the bread and wine into the Body and Blood of Jesus. Therefore, only someone who believes that such real change ("transubstantiation" for Catholics) has taken place is "discerning the Body" and capable of receiving it fruitfully, provided he satisfies the other requirements (having gone to the sacrament of Penance/Confession if aware of having commited a grave sin, and fasting for one hour prior). Which means that only the faithful of the Apostolic Churches can receive Communion at a Divine Liturgy celebrated by those Apostolic Churches.
On the other hand, ministers of Protestant ecclesial communities do not have apostolic succession and therefore do not effect the change of the bread and wine into the Body and Blood of Jesus, so that the bread and wine remain mere symbols of the Body and Blood of Jesus, exactly as Protestants believe. Therefore, when Protestants receive Communion at a Protestant service, they are not receiving it "without discerning the Body", because they think there is really no Body and that is exactly the actual case.