Why doesn’t Unitarian-Universalism grow at a faster pace? Top sociologists of religion Rodney Stark and Roger Finke think they know why:
All we are really saying is that, other things being equal, effort pays–that to the extent that organizations work harder, they are more successful. What could be more obvious? Moreover, at least in principle, the results of hard work are independent of theology. Thus, we are convinced that many of the declining American liberal denominations, for example, could grow if they could somehow get their current members to work at bringing in new members the way strict denominations do. Keep in mind that, although demand is smaller at the more liberal end of the spectrum than in the more moderate niches, liberal demand also is very underactivated. Hence, if liberal groups such as the Unitarians or Episcopalians could count on most members devoting four hours a month to door-to-door canvassing, they would grow a lot! The trouble is that these denominations are unable to motivate such efforts because in practice religious effort and theology are connected. Contrary to the complaints of disingenuous critics that our approach reduces religion to nothing but marketing, we have consistently argued that the inability of the liberal denominations to market themselves effectively is rooted in their doctrines–only vivid conceptions of an active and concerned supernatural can generate vigorous religious action.
Rodney Stark and Roger Finke. Acts of Faith: Explaining the Human Side of Religion. Berkeley: University of California Press, 2000: 257-258.