2 John 1:7-11 (ESV)
For many deceivers have gone out into the world, those who do not confess the coming of Jesus Christ in the flesh. Such a one is the deceiver and the antichrist.  Watch yourselves, so that you may not lose what we have worked for, but may win a full reward.  Everyone who goes on ahead and does not abide in the teaching of Christ, does not have God. Whoever abides in the teaching has both the Father and the Son.  If anyone comes to you and does not bring this teaching, do not receive him into your house or give him any greeting,  for whoever greets him takes part in his wicked works.
This is wholly ignored way to much these days. If more attention was paid to it closed communion would not even be a question. John is pretty clear here. Those who do not confess the coming of Jesus Christ in the flesh, are deceivers, we aren’t even supposed to greet them. So you can’t have Mormons, Jehovah Witnesses and the life communing at the table. We can’t recognize them as brothers.
Perhaps it is a bit murkier with those who confess the Trinity, so in essence confess the coming of Christ in the flesh. Yet, the reformed doctrine concerning Holy Communion is nothing less than a conclusion of a very bad Christology which in essence denies what they confess when they confess belief in the Triune God. And this confession cannot be tolerated in orthodox churches.
It becomes a hard thing to deal with over time. We all have friends and family of different confessions. They believe in Jesus Christ even if they are misguided. I don’t know that we have to question their salvation. Yet we have to recognize their false doctrine and the destructive effect it has on the body of Christ, and those who believe in him. The biggest problem with false doctrine is that ultimately it leads to unbelief.
“If anyone comes to you and does not bring this teaching, do not receive him into your house. And here we have to differentiate a bit from the teachers and the students. Many Baptists would know better with just a little teaching. But the Baptist Pastor, the teacher of baptistic doctrine, is going to have to answer for what he has taught, and we do not want to be a part of their wicked works. Have no doubt, false doctrine is wicked.