First, let me summarize: (1) God is the greatest being we can imagine. (2) To exist is greater than not to exist. (3) Therefore, God exists.
Forget, for the moment, that what this argument actually does is to prove that God is the greatest being that we can imagine and that, if we imagined God, we have to imagine that God exists. But imaging God's existence isn't proof that God does, in fact, exist. To address this we'd have to add premises like "to exist in reality is greater than to exist in the mind". Regardless, you get the basic idea.
You may also remember the argument has lots of critics. It's too simple. It's too clean. It's too cute.
Point in fact, I am one of those folks who finds the argument unpersuasive.
Today, though, I want to focus on something else. At its heart the ontological argument has us imagine a horizon of "greatness" and "perfection". The argument then goes on to say that existence must be, necessarily, a part of that vision. Maybe, maybe not. But in one sense it really doesn't matter. Because I think that horizon of "greatness" and "perfection" can do much of the work we want from any conception of God, with or without existence.
All of this becomes significant when I start thinking about what we mean by "the Kingdom of God".
When I think of "the Kingdom of God" what I have in mind is an ideal, a biblical ideal, a place where swords are beaten in plowshares and where the lion lays down with the lamb. Once posited, this ideal creates two eschatological functions, one negative and the other positive. Negatively, the Kingdom of God damns and stands in prophetic judgment of the status quo. In relation to the Kingdom of God the world is deemed fallen, sinful, and in need of rehabilitation. The horizon of the Kingdom of God creates the "oracle of the Lord", the "thus saith the Lord" of divine wrath and judgment.
Positively, the Kingdom of God functions as a goal state, a telos, a moral North Pole. A vision so beautiful, captivating, and wonderful that it infuses our dreams, our art and poetry, generation after generation.