Suppose I agree with theists that "God exists" is a necessary proposition, and
Interesting points. I take it that the most reasonable reply for a defender of the ontological argument to make is to claim that Prefoessor Smith's world is not in fact possible. If one can make a case for abstracta (properties or propositions necessarily existing) then there cannot be a world where only a single pencil exists. For a good case for such a Platonic position, see Roderick Chisholm's Person and Object. R.M. Adams also has a good discussion of the difficulty of imagining / conceiving of God's non-existence. I take this up in a modest book: Philosophy of Religion: A Beginner's Guide (Oneworld Press, Oxford) or in more detail in a discussion of Hume and necessity in Evidence and Faith: Philosophy and religion since the seventeenth century (Cambridge University Press).