"Anyone who goes too far and does not abide in the teaching of Christ, does not have God; the one who abides in the teaching, he 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, and do not give him a greeting; for the one who gives him a greeting participates in his evil deeds." (2 John 9-11)It is one thing to be wrong, it's another to be wandering in error. I suppose that if we took the time to examine our beliefs relative to one another we would find many areas where we disagree; areas in which one of us, if not both, are wrong. However, to take our error and to build upon it a theology that departs from the doctrines of Christ is a pathway to destruction for ourselves and our hearers. This is why Paul warns Timothy, "Pay close attention to yourself and to your teaching; persevere in these things, for as you do this you will ensure salvation both for yourself and for those who hear you." (1 Timothy 4:16) We may disagree with each other regarding what Jesus said and taught, but to depart from His teaching ensures our error and eventual condemnation before God.
There is safety in the Gospel message we have received. Jude refers to "the faith which was once for all handed down to the saints" (Jude 3) and that we "know all things once for all." (Jude 5) This does not mean that there is nothing more to learn and understand about God. However, it does mean that our search should be within the message already given rather than searching elsewhere for more knowledge and/or another revelation. I think this is what John means by "going too far." Some, not content with the message they have received, try to build upon it with "new" revelation until their new system of religion bears little resemblance to the message preached by Jesus and His apostles. Their search for something greater lead them astray. Their "new" revelation has set them outside the bounds of faith and into heresy. Whatever new knowledge and understanding we believe to have received from the Lord must fit inside the parameters of the message already given lest, we too, "go too far."
John warns of receiving such people into our company. Those who seek to draw people after themselves, to draw people after their own brand of teaching, seek to draw people away from Christ and from His church. They may claim enlightenment and to know to the path to holiness and to God, but it is a path that leads away from, not near to, God.
There was a certain heretic in the early church named Marion who taught many things contrary to the message of Christ. When Polycarp, Bishop of Smyrna, had a chance meeting with him in Rome, he walked past him refusing to acknowledge him. "And Polycarp himself replied to Marcion, who met him on one occasion, and said, 'Dost thou know me?' 'I do know thee, the first-born of Satan.'" (Irenaeus, Against Heresy, Book III, chapter 3) I do not suggest that this should become our normal greeting for people in the marketplace, "You are the first-born of Satan!" but the early church took this warning of John seriously and understood the those who sought to compromise the church were the enemies of the church and sought her harm not her good. We should be wary of such people and keep ourselves clean of their company.