This post brought to you by caraboska, sort of. :)
The subject came up in some posts caraboska's made recently, so that's why it's brought to you by her, because it's made me think about it, which I normally don't, really.
See, I grew up in Missouri Synod Lutheranism, which is the 'high church' version of Lutheranism. Fairly similar to Roman Catholicism, in many ways, actually. So there's a lot there that's simply ingrained as the 'right way' to do things.
One of these things is closed communion. For those who don't exactly know what that is (I doubt there're any who don't, but I cover my bases), closed communion means that one may only receive communion in that church if one is a member of that faith. So a Missouri Synod Lutheran would only ever receive communion in a MS Lutheran Church (It's possible that some 'crossover' communion is possible with other Lutheran synods, but that's not how we were taught growing up. Those 'other' Lutherans were all wrong and degenerating into ecumenism anyway...), and anyone who is not a MS Lutheran is not allowed to receive communion at a MS Lutheran church. The same goes for Roman Catholicism. You're *only* to receive if you're Roman Catholic. Not that they check i.d. card's or anything, but that's the rule.
Now, Orthodoxy also has closed communion, to people who are not Orthodox (I'm not sure if there're any Orthodox churches that don't 'cross' commune. For instance, I believe that one can be Russian Orthodox and receive in a Greek Orthodox parish, etc. But I could be wrong here.), but they do sort of check i.d. cards, from what I understand. It's taken very seriously, is my point there.
Anyway, the concept has never really bugged me, in part because well, it's what I grew up with, and in part because it makes perfect sense. Communion, to me, is not merely about the act of receiving the Body and the Blood because I believe that they are, in fact, the Body and the Blood. When you receive communion, it's attesting to the fact that you believe, with the rest of the Church, in the Church - the theology, the doctrines, the history, everything. And not just with those present with you right then and there, but all over the world, and throughout time. In heaven as well. You are in *union* with all the other believers. And closed communion keeps that unity, and safeguards it.
I've never taken communion in a church that wasn't MS Lutheran or Roman Catholic. And I only received in those churches when I was in union with them. My parents attended a Mennonite church for several years, and I went with them for a while, but I would never, ever take communion with them. Because we don't believe the same things, and I would never want to imply union with them by communing with them, see?