Thermostat or Thermometer?

Relevant to today's holiday is this, taken from a 1965 interview with Dr. King:

[T]he most pervasive mistake I have made was in believing that because our cause was just, we could be sure that the white ministers of the South, once their Christian consciences were challenged, would rise to our aid.... The projection of a social gospel, in my opinion, is the true witness of a Christian life.... The church once changed society. It was then a thermostat of society. But today I feel that too much of the church is merely a thermometer, which measures rather than molds popular opinion.

(Thanks to Professor Friedman.)