I heard one of the best arguments against open communion the other day. I was listening to Catholic Answers Live on the radio (great show by the way--comes on from 5-7 pm central time on EWTN Global Catholic Radio Network), and this person called in wondering why the Church did not allow open communion with other Christians. His point (which was a very good one) was that the Church was always saying how it desired there to be one church and all Christians to be united, yet it was very exclusive in the distribution of communion. Only practicing Catholics are allowed to take the Eucharist.
Well, there are the standard reasons we say in response to this question: you should not receive the true Body and Blood of Christ unless you truly believe and understand (to human capacity) the mystery that it is the Real Presence of Christ Himself, otherwise you eat and drink unworthily and are guilty of Christs body and blood (which Paul warns against in 1 Corinthians 11:27); and when we go to Communion we are proclaiming that we are in union with the Catholic Church and believe what it teaches, so, for a Protestant to receive would be a lie. So the apologist gave the caller these standard responses and the caller's rebuttal (also a good one) was that it seems we are actually working against striving for this union of the church because we are not allowing others to partake in our Eucharist--we are pointing them out as divided against us.
So, the great response that this apologist (and I regret not remembering his name) gave was simple. He said that we would LOVE to have everyone come to our communion, but that does not mean we should pretend there is a union there that doesn't exist. In other words, we should not assume that these people are in communion with us just so they won't feel left out. There is a separation that needs healing, and we can't act like it isn't there, otherwise it would never heal. That would be like trying to cure cancer by pretending you don't have it. It doesn't work that way. If we allowed everyone to receive communion in our church we would be saying that it is okay that they don't believe what the Catholic Church teaches and the divide would continue. The reality is it's not okay that many do not believe in the Catholic Church. It is absolutely not okay.
I'm not saying that Protestants are all bad or that they don't have the truth--after all, they are Christian--but they are missing some important parts of the whole Truth that can only be found in the Catholic Church. I believe in the Catholic Church because I believe its the whole TRUTH. If I didn't believe that, then why would I even be Catholic at all? I would have no true faith because it would be okay to believe whatever was convenient. There are over 33,000 Protestant denominations (World Christian Encyclopedia, 2nd edition, David Barrett, 2001), each believing something different, and many of them think that doesn't matter as long as you're Christian. What kind of faith is that? If you believe one set of doctrines and someone else believes another and you're fine with that, then how can you say that you believe in the Truth? That's relativism.
The fact that the Catholic Church says it's NOT OKAY that we are divided and makes a point that you must be united with the Catholic Church in order to receive the Eucharist (the greatest Sacrament), says a lot about the faith of the people. We believe that we are the Church that Christ instituted and He never ever intended it to be divided (John 17:20-23). We want to heal the divide; open communion would only make it worse by saying it's okay to believe what you want. Instead, we must evangelize to our Protestant brethren and draw people into the faith. The Eucharist is for everyone, but you must have true faith in it. We would LOVE to have EVERYONE participate in our communion, but that doesn't mean we are going to compromise. Communion is for All, but if you truly want it, you must come to believe in the Truth, passed on to us by the Church established by Jesus Christ--the Catholic Church.