Lewis’s recipient – John Coker – was a student of Alvin Plantinga at Notre Dame in the 1980s. Coker wrote a paper arguing that modal realism, with Lewis’s added trappings of concrete worlds, no overlap, qualified principle of recombination, etc, is atheististic in the sense that a theist has reason not to believe Lewis’s theory of possible worlds.
As Lewis mentions in his Introduction to Philosophical Papers I (1983, p. xi), he states what atheism and theism amount to given modal realism. As an atheist and modal realist Lewis admits there is a god, in fact, there are many gods, but none of them are actual (no god or gods are among our worldmates). The theist working within modal realism is forced to say that any god-candidate is not omniscient, or omnipotent, speaking with quantifiers wide open. As Lewis says in this letter God has been belittled. The important trait that God no longer has is omnipotence. The problem for the theist is that any god-candidate does not fit the description of the theist’s object of worship – worship of an all-powerful entity.
Interestingly, Lewis realises that the theist’s position affects the dialectic of his defence of modal realism. One great challenge for Lewis is that his view goes against common sense, something that he thinks we should be faithful to as much as we can. While he admits in one sense that modal realism violates common sense, most of chapter 2 of On the Plurality of Worlds is an argument to mitigate the idea that the modal realist must revise his or her extant beliefs. His strategy is to reduce the anti-common-sense objection to an incredulous stare (that turns on a certain interpretation of what ordinary people mean by ‘actual’). But what this letter brings out into the open is that this argument is relative to *Lewis* and his religious commitments. *Lewis* can believe in flying pigs and other wild possibilia and carry on living his life as before. Can *you*?