If we are honest we can respect that there has always been a tension between how inclusive and how exclusive to be regarding the company we keep. We see this tension in Mark 9, Matthew 12, Luke 11, and even Numbers 11.
Moses asked those who were worried about rival prophets, this rather rhetorical question "Are you jealous for my sake?", and Jesus, flowing in the same spirit, answered ""For whoever is not against us, is for us."
I think we need to be merciful with others and ourselves with this tension, and admit that the lines we use to exclude others are not always as clear as we would like them to be. This is why i think denominations emerge in the first place, to allow people of like mind to draw the smallest circle around themselves, in order to resolve as many controversies in their own consciences as possible. In this light, I think denominations can be a healthy decision if only to relieve some tension for ourselves, but it becomes yet another source of bigoted unhealthy tension if we must set out to prove that everyone else is a heretic for not ascribing to our particular strain of the faith.
It is natural for people to want to resolve dissonances in their own ears, because we are comforted by harmonies. Likewise it is natural for people to want to align one another's beliefs to their own; so much so, that I imagine many readers of my post will become uncomfortable with the ambiguity I am suggesting we accept here. Yet I am not ambiguous on one point, and that is we who confess Christ, are held secure in our belief by his perfect theology, not ours. The government is on his capable shoulders, thankfully.
And so, seeing as not even Jesus' desciples were perfectly aligned with their masters' beliefs and world views, it is likely that the "rivals" they were fretting over also held to some some unorthodox points in their frameworks. In response, Jesus neither tried to recruit them to his group, nor did he condemn them for having their own fellowship. He only offered the curiously ambiguous phrase, "those who are not against us, are for us." Perhaps his desciples wanted to build an institution, while Jesus simply wanted to build his true Church.
In one very real sense, all of us come from different institutions, but we share a common experience; we are all survivors of those various failed institutions we have put our faith in. Some of us were Catholic, and some of us were Evangelical, some of us were Jewish, some of us were Muslem, and even some of us were academics! Now, we are seeing our once stable institutions, both secular and religious, crack and crumble all around us. I admit it is frightening to consider that even our own institutions are toppling and have already fallen into the piles of history. Within the rubble of these fallen structures are going to be survivors. Rather then rope off the wreckage, why not be open to being kind to those who, who like us, see through a glass darkly, but still hope to find some light.
Just my thoughts on this important question,
Blessings,
MC