Hillel teaches in Pirke Avot (2:4): do not separate yourself from the community. I'm not a historian, but I have the impression that at the time he was writing, towns tended to have one (Jewish) community. The shul you go to and the shul across the street that you wouldn't set foot in seems like a newer idea.
I'm a 21st-century city-dweller. There are at least a dozen shuls within a mile of my house, to say nothing of the chavurot and other groups that I don't know about. What exactly is the community that I must not separate myself from?
Am I separating myself from the community if I:
Move from one synagogue to another, within the same "flavor" of Judaism? (e.g. I move from one Modern Orthodox shul to another.)
Move from one synagogue to another, moving to a different "flavor"? (e.g. I'm Modern Orthodox and conclude that what really speaks to me is Lubavich.)
Cease affiliating but join or form a chavurah?
Remain affiliated but stop going? (I think I know the answer to this one, but just checking.)
In all of these cases I've separated myself from a community, but in three of the four I've then joined another (not necessarily of the same size). According to Hillel, am I behaving inappropriately?
Disclaimer: this is a thought experiment. I have no intentions of leaving my current synagogue (and by the way, it's neither Modern Orthodox nor Lubavich).