I wonder if it is time to dispense with the work "denomination".
As a member of the Church of the Nazarene, and gladly so, I see what is called a denomination as something more akin to a movement. I’m sure there are plenty of people who immediately shudder at the use of the word "movement", but I find that the work denomination adds to the separation between God’s people.
The Church of the Nazarene, for example, places itself under the "Wesleyan-Holiness" umbrella, which was a movement. It seems that much of what separates the church (specifically, the so-called Protestant section) is not core salvation issues (although some would disagree), but spiritual movements that have moved individuals to express their faith.
The Roman Catholic church, for example, has different "schools". From what I understand, much of what makes them different (Jesuits, Hospitallers, Carmelites, Benedictines, etc.) from other Roman Catholics is what "moved" them to express their faith. Some of them have developed additional layers of theologies, and others developed rules to live by.
Some Roman Catholic apologists point the finger at Martin Luther starting the disintegration of the Roman Catholic church, and the seemingly resulting infinite number of "denominations". However, the Roman Catholic church has its own historical issues in that area, specifically the split from the (Eastern) Orthodox church (and there was plenty of blame to go around on that).
The Church of the Nazarene is young as "denominations" go (est. 1904). One of the things it has done well is patiently (biblically, I’d say, long-sufferingly) work on keeping the theological tent as large as possible. That takes determination and humility.
Perhaps, that is the key. It takes humility to accept "movements", and pride creates denominations.