What do you mean that nothing can be known for certain? Do you mean about God, or about anything at all?
About anything unseen. God, aliens, bigfoot, etc. Its best to say "I don't really know for a certainty if they exist".
I agree that we don't know for certain whether there is any sort of ethereal consciousness we can pray to or communicate with. But I think certainty about the existence of God depends on how you define "God."