“As to the word itself, it is generally allowed to be of Greek extraction. But whence the Greek word, enthousiasmos, is derived, none has yet been able to show. Some have endeavoured to derive it from en theoi, in God; because all enthusiasm has reference to him. … It is not improbable, that one reason why this uncouth word has been retained in so many languages was, because men were not better agreed concerning the meaning than concerning the derivation of it. They therefore adopted the Greek word, because they did not understand it: they did not translate it into their own tongues, because they knew not how to translate it; it having been always a word of a loose, uncertain sense, to which no determinate meaning was affixed.
It is not, therefore, at all surprising, that it is so variously taken at this day; different persons understanding it in different senses, quite inconsistent with each other. Some take it in a good sense, for a divine impulse or impression, superior to all the natural faculties, and suspending, for the time, either in whole or in part, both the reason and the outward senses. In this meaning of the word, both the Prophets of old, and the Apostles, were proper enthusiasts; being, at divers times, so filled with the Spirit, and so influenced by Him who dwelt in their hearts, that the exercise of their own reason, their senses, and all their natural faculties, being suspended, they were wholly actuated by the power of God, and “spake” only “as they were moved by the Holy Ghost.”
Others take the word in an indifferent sense, such as is neither morally good nor evil: thus they speak of the enthusiasm of the poets; of Homer and Virgil in particular. And this a late eminent writer extends so far as to assert, there is no man excellent in his profession, whatsoever it be, who has not in his temper a strong tincture of enthusiasm. By enthusiasm these appear to understand, all uncommon vigour of thought, a peculiar fervour of spirit, a vivacity and strength not to be found in common men; elevating the soul to greater and higher things than cool reason could have attained.
But neither of these is the sense wherein the word “enthusiasm” is most usually understood. The generality of men, if no farther agreed, at least agree thus far concerning it, that it is something evil: and this is plainly the sentiment of all those who call the religion of the heart “enthusiasm.” Accordingly, I shall take it in the following pages, as an evil; a misfortune, if not a fault. As to the nature of enthusiasm, it is, undoubtedly a disorder of the mind; and such a disorder as greatly hinders the exercise of reason. Nay, sometimes it wholly sets it aside: it not only dims but shuts the eyes of the understanding. It may, therefore, well be accounted a species of madness; of madness rather than of folly: seeing a fool is properly one who draws wrong conclusions from right premisses; whereas a madman draws right conclusions, but from wrong premisses. And so does an enthusiast suppose his premisses true, and his conclusions would necessarily follow. But here lies his mistake: his premisses are false. He imagines himself to be what he is not: and therefore, setting out wrong, the farther he goes, the more he wanders out of the way.”

Sermon 37 "The Nature of Enthusiasm"
Sermons on Several Occasions (1771)