The term ‘enthusiasm’ is used by John Locke in Book IV, Chapter 19 of An Essay Concerning Human Understanding to describe the psychological state of being disposed to accept a belief as true, in the absence of evidence, because it feels true.
Locke believes there are only two legitimate sources for belief formation: reason (which he calls “natural revelation”) and God (what he calls “genuine revelation”). Since God has equipped us with natural faculties of reason (i.e., the five senses and a rational mind with which to process information) he will not directly reveal true beliefs that could be known via “natural revelation”. Therefore, the kinds of beliefs that God does directly reveal to the mind are limited to those beliefs that are true, but beyond the ordinary capacity of human reason (divine truths).
But just because God has given us rational faculties does guarantee that we will use those faculties the way he intended. Because we have free will, we have the capacity to formulate and hold beliefs that are not rationally justified; that is, we have the capacity to believe things that are derived neither from divine revelation or reason. Enthusiasm refers to one type of unjustified belief that people might hold. DY>