Think in terms of possible worlds: one could imagine (and some people on here probably do, wistfully) a world in which I never existed, and ditto for anyone and anything else. If there are an infinite number of alternative universes, then such universes presumably exist. However, one can't imagine a situation in which God exists in some universes but not in others: if there's a God then God made the whole multi-shebang, and therefore exists in all possible universes. On the other hand, if God does not exist in this universe, God doesn't exist in any. In that sense, God is necessary - if God exists at all. The question of God's existence, however, is left entirely open.