"Oasis" is actually a pretty good metaphor for how some idealize the Church. But it doesn't suggest unity or cohesion. The diverse, hostile species that frequent oases to get basic needs met don't usually form communities. 

How we define what it means to be an Adventist won't really address the problem of finding shared deep commitment while clinging to our diverse identities and values. Did early Adventists really have a "loose set of shared beliefs?" That's not my sense. My sense is that they were prepared to sacrifice their lives and "fortunes" to advance their beliefs. 

And that selfless, non-institutionalized identity had the same built-in expiration date as all pre-institutional movements. Without the institutional container, there would be no SDA Church. Of course there might instead be thousands of vibrant Sabbath keeping, Advent preaching movements with diverse theologies. But internally, they would each probably be less diverse than the current SDA denomination.