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Is it alright for an Orthodox christian to go into a Catholic church and vice versa? That is:
Does either Eastern Orthodox or Catholic teaching object to occasional participation of an Eastern Orthodox Christian in a Catholic service (Mass)? What about occasional participation of a Catholic in an Eastern Orthodox service (Divine Liturgy)?
From a Catholic point of view, there is no problem with (a) a Roman Catholic attending an Orthodox Divine Liturgy and receiving communion or (b) an Orthodox attending a Roman Catholic mass and receiving communion. This is, however, restricted to circumstances when a Roman Catholic mass is unavailable.
The Orthodox position is different; Roman Catholics would not be permitted to receive communion in an Orthodox church and Orthodox would not be permitted to receive in a Roman Catholic church.
I've had surprising difficulty finding documentation for this, but here's an EWTN article explaining the Catholic position and here's an Orthodox Church of America page explaining the Orthodox position. They aren't exactly the best sources imaginable, but they are accurate in my experience.
The Eastern Orthodox church and the Catholic church are two separate churches. The Eastern Orthodox church is not the same as the Eastern Rite Catholic churches, whose bishops are in union with the bishop of Rome.
The Catholic church views the leadership and the sacraments as valid, since they derive from apostolic authority and the Eastern Orthodox church can trace a valid succession all the way back to the Apostles (unlike Anglicanism, where at one point Anglicans clearly intended to no longer make Catholic priests and bishops, but to make Anglican priests and Bishops). As such, they are considered a true "Church", not a schismatic or heretical community.
A member of the Catholic church would be allowed to participate in all ways that would be considered honest ecumenism. There is no reason why one could not join with them to pray, sing, read scripture, attend a wedding or baptism, etc.
However, even though their sacraments are valid, it is not licit, or allowed, for a Catholic to participate in the sacraments of the Eastern Orthodox church. Primarily, this is because they are not a member of that community, and in the true spirit of ecumenism, a Catholic would not want to present themselves for Eucharist, for example, because the Eastern Orthodox churches may not always not intend to provide its sacraments to non-members.
Additionally, when Catholics receive Communion, they are expressing unity both with Christ and with the Catholic church. As a result, when a Catholic liturgy is available, this is understood as the proper place for Catholics to worship and receive.
However, when a Catholic liturgy is unavailable, a Catholic is permitted to attend services and receive in an Orthodox church. Ideally, the parishioner would seek the permission of both priests, to avoid any confusion, especially if the situation is not a one-time event.
So it depends on the nature of the participation. For example, if a Catholic is married to an Orthodox spouse, the Catholic would be allowed to attend and participate, but not communicate, in an Orthodox liturgy on a regular basis.
