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][1] 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][2] 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.


  [1]: http://www.cnrtl.fr/definition/d%C3%A9isme
  [2]: http://www.cnrtl.fr/definition/th%C3%A9isme