Leaving aside for the moment fulfilment of scripture, miracles and other signs which indicate a God, surely the raw probability of God existing must be 0, because probability is a calculation based on the expected outcomes. Since God is supernatural, he is not an expected outcome.
This is not to say that he does not exist, but simply that there cannot be a probability assigned to his existance.
For since the creation of the world God's invisible qualities—his eternal power and divine nature—have been clearly seen, being understood from what has been made, so that men are without excuse.
Romans 1:20
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