A “theic” is someone who is addicted to the immoderate use of tea; a tea-drunkard.

Thus, as this guide to tea drinkers’ disease warns:

The predominance of nervous symptoms is a characteristic of theism; general excitation of the functions of the nervous system may be observed; or the weakness may be noted more especially in the brain as distinguished form the spinal cord.

This, of course, explains the present plight of the Church of England: all the vicars are drowning in tea and, with their weakened brains, have abandoned one theism for another.