—2 John 9 (NASB)
There IS a line we must not cross, a doctrinal threshhold that can be practically measured in this life indicating whether or not one is maintaining a right relationship with God: Whether or not we "abide in the teaching of Christ".
"Abide" means to stay, to remain, to live. Its vocabulary cousin is "abode", which is the place in which one daily dwells or lives. The boundaries of our "abode" is the teaching of Christ; straying beyond takes us to a new place of residence, no longer living under His roof.
John qualifies this statement with "anyone", not with a personal pronoun such as "you". It applies to us personally, but we are also to apply it corporately. We not only have a personal responsibility to always remain true to the teaching of Christ and never stray, but we also have the responsibility of providing feedback to our brothers and sisters in Christ that no matter how well-intentioned, they have crossed a line that matters. In effect, they are leaving the Father and the Son both behind.
It's not acceptable to say we love Jesus with the mouth only and not follow His Word from the heart. It's not enough to acknowledge all the goodness and positive attributes of biblical teaching without actually putting it into practice. It's not good enough to live "near" Christ or to have Him "close by". In all these instances, we have gone "too far" and no longer "abide in the teaching of Christ". This isn't tolerable personally, nor for the rest of the members of the body. What are you going to do in regards to those you know have gone too far, and no longer live according to God's Word?†††