I.e., Monygham is a devoted man who happens to be a Ribierist. "Devoted" meaning, of course, consumed by a dream-ideal, namely his devotion to Mrs Gould.

Ribierism, we recall, is the movement of national reform, the political equivalent of Mrs Gould's altruism, and the political expression of exactly the futile, false hope that the Devil is cynically trying to re-establish.