can someone explain to me in a way a non Catholic could understand why we can't attend their Sunday service? Thank uWhich is a question I think many readers might be able to relate to. Once upon a time, Catholics either lived in overwhelmingly Catholic countries (think of Catholic Europe or Latin America) or, especially here in America during the immigration boom of the late nineteenth - early twentieth centuries, Catholics tended to live in "Catholic ghettos" and mostly associated with other Catholics. Such is no longer the case and Catholics, many of whom marry someone of another faith, may often find themselves invited to attend a non-Catholic Sunday service. What, then, is a Catholic to do (short of shouting "heresy most foul" and sprinkling the non-Catholic with holy water).
The first question might be whether we even have a choice as Catholics at all. Can a Catholic go to service at a non-Catholic church or ecclesial communion1? The answer differs between the two groups, but as most of my readers will be asked not to attend the Divine Liturgy of a separated Eastern Orthodox Church, we'll focus on the "ecclesial communions," Protestant denominations and non-demoniational groups. The simple answer is, yes. A Catholic is perfectly free to attend a Protestant service if they choose to. There might be valid reasons for not wanting to attend, such as the fear of scandal - i.e. giving the impression that it is a matter of indifference whether one is Catholic or Protestant. This might be more or less of a concern depending on who we will be attending the service with, how well they know us, and whether or not we've directly addressed the issue. The service might also be virulently anti-Catholic, the kind that frequently calls Mother Church the "whore of Babylon."
On the other hand, there might well be good reasons to attend. It might be an evangelical opportunity, maybe we're attending the Protestant service and the following week our Protestant friend will attend Mass in return. It might also be an example of authentic ecumenism, perhaps the service is a part of a series of prayers for the end of the horror of abortion during the March for Life. More likely, the service might be a baptism of family member who is not Catholic. Converts are likely to find themselves invited to many of these services.
With good reasons being possible for both attending or declining to attend a Protestant service, each Catholic is free to make a prudential decision on whether attending this particular service at this particular time is the right decision. No sin is involved either way.
Which brings us back to the original question, assuming a Catholic doesn't want to attend the non-Catholic Sunday service and assuming they don't want to seem rude, what might be the best way to "get out of" going?
Probably the best answer is to just explain to the non-Catholic why you don't want to attend the service. If the Church is rabidly anti-Catholic this could be relatively easy. If the Catholic is new in the Faith or has a poor understanding of her Sacred Doctrine and doesn't feel comfortable with the idea of encountering false teaching (which may or may not come up at the non-Catholic service - or, if we are being honest, even at some Catholic parishes), that also could be explained, although the chance of upsetting the non-Catholic increases.
If being direct is too likely to cause problems the best route may be just pointing out that, as Catholics, we are bound to attend the Holy Sacrifice of the Mass on Sunday. Going to a Protestant service doesn't "count;" it doesn't satisfy our obligation to attend Mass. Thus, if we choose to attend the Protestant service, we'll also have to attend Mass that day. While the Protestant might not understand this, they probably would understand your unwillingness to attend Mass and the Protestant service in the same day. You can even ask them if they want to go to their service and then Mass later that day with you. They probably won't and probably won't be offended either.
1. "The Churches which, while not existing in perfect communion with the Catholic Church, remain united to her by means of the closest bonds, that is, by apostolic succession and a valid Eucharist, are true particular Churches....ecclesial communities which have not preserved the valid Episcopate and the genuine and integral substance of the Eucharistic mystery, are not Churches in the proper sense; however, those who are baptized in these communities are, by Baptism, incorporated in Christ and thus are in a certain communion, albeit imperfect, with the Church." Dominus Iesus, 17