The downside of diversity, as I noted yesterday, is that it reduces social capital and group trust. People are more likely to invest themselves deeply and give sacrificially to a group with which they share the deepest things in common.
Churches grow best and most explosively when they are based on cells of people who are like one another, who share the deepest levels of culture. Booming megachurches may be made of all kinds on Sunday morning, but they divide into cells of similar kinds on Wednesday night. This is called the homogeneous unit principle. It is the church-growth version of "birds of a feather flock together."
Churches are more like families than they are like workplaces. That is, they are more like primary groups than secondary groups. And the more a church works like a family, the stronger it will be and the more likely it is to grow.
Big denominations can be very diverse. Big congregations can be somewhat diverse. But strong church cells -- the ecclesiola in ecclesia, however constructed -- need to be similar in the most important ways.