The question was posed to me by a friend of mine. I know the answer from a Catholic perspective–it is a sign of unity of Faith and if you don’t believe what all the Catholic Church teaches then you should not partake, because “anyone who eats and drinks without discerning the body, eats and drinks judgment on himself.” --1 Cor. 11:29
This friend in a Lutheran, and he believes in the True Presence, yet they have open communion at his church. He believes that if someone were to go to Communion and believe, Jesus would be present, but if they didn’t believe, He would not be.
My response to this was to say Jesus is present no matter what because it is a real presence, just as a tree in a forest is real whether or not a person goes to see it or even knows its there.
But his frustration remains. The argument is that the Catholic Church claims that we are trying to unify all churches into one, yet we are being divisive by excluding others at our communion. My friends stance is he would never be a part of a church that was so exclusive.