In its article 118, the Ecumenical Directory (ED) states:
In liturgical celebrations taking place in other Churches and ecclesial Communities, Catholics are encouraged to take part in the psalms, responses, hymns and common actions of the Church in which they are guests.
This seems to be self-evident, but it is more complex than one might tend to believe. When you – as a Catholic – join a non-Catholic service where psalms are jointly read or sung and the text version is a bit different to the translation you are most used to, it needs concentration and effort to focus on that strange text. Gestures like standing, sitting, kneeling, the sign of the cross, blessing signs, etc. can even be provocative to one’s personal piety: mainly when you are used to assigning certain gestures to certain theological meanings or liturgical acts. The entire habit of physical movement in Eastern churches is somehow different to the Western way of dealing with one’s body during a liturgy: that is why Easterners can easily identify Western guests after just a few seconds. For me, the most provocative challenge is when I am invited to a charismatic worship service – by whatever denomination – where I am supposed to raise my arms, jubilate spontaneously and speak or pray extempore.
Anyway, it is not so easy to “take part in common actions” of the host community, and I should not blame myself if I fail. Liturgy does not mean doing things once and never again: it is more of a custom, a culture that you have to get used to. There is nothing bad about being a guest. But the ED says: Do not be too Roman when you are a guest in a different tradition. Respect the customs, try to adopt them and try to learn from them. They are not your enemies, they can be an inspiration!
And then there is an even more serious question: What if it is the other church’s custom to invite all Christians – or even non-Christians – to take part in the Eucharist? I have heard such explicit invitations several times (in Episcopalian, Lutheran, and Old Catholic communities), even with further explanations: “Everyone who believes that this is the body and blood of Christ is invited,” “everyone who is baptized is invited,” or the most touching invitation I have ever heard, “everyone who knows what it is like to be hurt and be in need of healing is invited.”
Of course, there is a solution by the Canon Law: Can. 844 CIC does not permit Catholics to receive the Eucharist in non-Catholic churches (with some exceptions that do not seem to apply to my question). Now you can simply argue that the CIC outweighs the ED. Nevertheless, theological questions cannot (or should not) be reduced to legal questions, and the Canon Law can be changed if the legislator decides to do so. If Ecumenism is a core principle of the Catholic Church, and if Ecumenism – among other things – means being a respectful guest when a Catholic is invited by another Christian tradition, would it not be theologically appropriate to accept an invitation to the Eucharist? This does not automatically mean that the Catholic Church would be bound to do the same with their guests. It would also not imply any dogmatic declaration about the Eucharist. It would only mean the acceptance of a host from whom I might be enriched. I do not simply want to say that the ED outweighs the CIC in my eyes. But as far as I can see all theological debates about intercommunion are reduced to dogmatic matters about the Eucharistic substance and the ordained ministry. What would happen if we also took account of the relation between host and guest in Ecumenical encounters?