According to Anselm, I'm a fool. I believe God exists only in the imagination. But even Anselm grants that God does exist in my imagination and that I have a grasp of what things would have been like had there been a God. Had there been a God, for example, God would have been very displeased with Hitler and commanded him to stop.
More generally, it's daft to think that God's existence is necessary to ground normative ideals, because the whole point of ideals is that they float free from the mess of our actual reality. The question of how things should be does not fundamentally depend on how things in fact are. Ideal standards can be grounded in counterfactuals, e.g. facts about what an ideal spectator would recommend; whether such an ideal spectator actually exists in the here and now is, quite simply, irrelevant. (This is a familiar point: one may ask, "What would Jesus do?" without requiring that Jesus actually be in that situation.)