From Wels.net

Close or closed Communion is not just some strange "WELS synodical rule." Closed Communion is a biblically faithful practice that testifies to the fact that, where there is not a common confession of faith in all that Scripture has to say, we cannot stand or kneel together at the Lord's table as if differences in teaching are of no importance. Because the Scripture teaches us of our salvation and guides us in living to our Savior's glory, we have learned to "hate every wrong path" (Psalm 119:104). When we commune together, we are making a statement of unity in the faith. That confession becomes an empty shell of itself if we do not faithfully practice close or closed communion.

Of course, we must be careful, as much as we can, that people understand what we are saying. At times, visitors and guests seem to get the impression that we are making a judgment on their heart (whether they are a Christian or not). Instead, we are doing what Scripture asks us to do, and that is to be discerning judges of that which people confess to believe as we compare that to Scripture.

What often adds to the potential for confusion and taking offense by worship guests is that most of the visible church neither practices closed Communion nor cares much anymore about being careful with biblical doctrine. We are struggling against the religious spirit of the times in which we live.

Finally, even where we have been patient and careful and loving in our explanation, we cannot help that at times people will become upset and angry at our practice... That should not surprise us. Our proud natural human hearts--which even Christians will struggle with until the day we go home to heaven--often take offense at biblical truth.

Grace is not common.