John is not talking about errorists who come to your house on business. The is about fellowship in Christ, receiving them as if they are Christians as you are.
I know a man who whose friend died, a friend he had been witnessing to and had hopes that they had true faith. Somehow a neighbour got involved who was a Catholic and arranged for the funeral to be in their church carried out by the priest. He decided to go to the service out of loyalty to the friend, but he was completely out of sympathy with all that took place there. At one point the priests asked everyone in the congregation to shake hands with the person next to them. That was going too far and the man would not do it. When the priest came to his row and tried to shake hands saying, ‘Peace be with you’, the man replied, ‘There is no peace between us’ and would not shake hands. The priests was deeply offended and spoke to him afterwards. The man told him that the dead woman was certainly no Catholic.