Why are "theism" and "deism" different?

Solution 1:

I don't think there is any etymological explanation of the difference in meaning and usage between "deism" and "theism". If there is one, it doesn't seem to be very obvious, as you have already noticed.

Different words often naturally develop different connotations, and when they don't, people often put them to use for different concepts or try to figure out some way to distinguish them (or sometimes, one of them just falls out of common use). I think this is probably the main reason why "theism" and "deism" have distinct meanings in present-day English.

Basic etymological facts

"Deism" and "theism" both seem to have been coined sometime in the second half of the 2nd millenium. The Oxford English Dictionary (OED)'s earliest example of deism in English is from 1682; the OED further indicates that the word was taken from French. The French CNRTL indicates the earliest source where the French déisme is known to have been used is "1662 (Pasc., Pens., part. II, art. 4 ds Littré)". The French word for "God", dieu/Dieu, is related to the Latin stem used in déisme.

The OED's earliest example of theism in English is from 1678. It says théisme was used in French by Voltaire (who lived 1694–1778), but it doesn't say in what context. The French CNRTL says théisme was actually taken into French from English, and gives "1745 (Diderot, Principes de la philosophie morale, ou essai de M. S** A. Cooper Comte de [Shaftesbury] sur le mérite et la vertu, avec réflexions, Amsterdam, p. 12, note)" as the earliest known example in French.

My thoughts and conjectures

Theism and deism are fairly "jargon"-y words. Jargon doesn't have to have a logical etymology; the point is to have any kind of label at all. E.g. the prefixes "a" and "im-" usually mean the same thing, but people have come up with a special distinction between "amoral" and "immoral".

So I would say there is no etymological explanation. The Oxford English Dictionary entry for "deism" cites a source that says that:

1877 E. R. Conder Basis of Faith i. 25 Deism should etymologically have the same sense with Theism, but it is commonly taken to carry with it the denial of what is called revealed religion. Theism conveys no such implication.

Solution 2:

Simplistically, deist (and hence deism) derives from Zeus while theist (and hence theism) derives from the more generic theos, both terms of Greek extraction:

deist (n.) 1620s, from French déiste, from Latin deus (see Zeus). Related: Deistic (1795). Also see deism. - etymonline

theist (n.) 1660s, from Greek theos "god" (from PIE root *dhes-, forming words for religious concepts) + -ist. The original senses was that later reserved to deist: "one who believes in a transcendent god but denies revelation." Later in 18c. theist was contrasted with deist, as believing in a personal God and allowing the possibility of revelation. - etymonline

The term deism goes back to the name Zeus, which in turn goes back to a PIE root meaning to shine. Further:

However, the word "deism", as it is understood today, is generally used to refer to the movement toward natural theology or freethinking that occurred in 17th-century Europe, and specifically in Britain. - wikipedia

It might be that 'theologians' (using that term loosely, given the distinction deists made with respect to theism) of the enlightenment movement (which championed reasoning) picked the term deism for its link to the notion of 'light' embedded in the movement's name The Enlightenment. There is also some (very slight) circumstantial support in the "father of lights" term attributed to someone said to be a deist:

Deist authors – and 17th- and 18th-century theologians in general – referred to God using a variety of vivid circumlocutions such as ... Father of Lights. Benjamin Franklin used this terminology when proposing that meetings of the Constitutional Convention begin with prayers - wikipedia

The term theist has a broader scope:

The term theism was first used by Ralph Cudworth (1617–1688).8 In Cudworth's definition, they are "strictly and properly called Theists, who affirm, that a perfectly conscious understanding being, or mind, existing of itself from eternity, was the cause of all other things". - wikipedia, citing Cudworth, Ralph (1678). The True Intellectual System of the Universe, Vol. I. New York: Gould & Newman, 1837, p. 267.

It would make sense that the distinction between deism and theism solidified due to the philosophical polarisation between the concepts of an involved God and that of one uninvolved:

Prior to the 17th Century the terms ["deism" and "deist"] were used interchangeably with the terms "theism" and "theist", respectively. ... Theologians and philosophers of the 17th Century began to give a different signification to the words... Both [theists and deists] asserted belief in one supreme God, the Creator... . But the theist taught that God remained actively interested in and operative in the world which he had made, whereas the Deist maintained that God endowed the world at creation with self-sustaining and self-acting powers and then surrendered it wholly to the operation of these powers acting as second causes. - wikipedia, citing: Orr, John (1934). English Deism: Its Roots and Its Fruits. Eerdmans. p. 13.