Simply say to the theist:
(1) Theists don't observe themselves as omniscient.
(2) Theists 'want to' absolutely believe that God exists. (Impossible, due to (1), ie theists lack absolute awareness)
(3) Theists don't probably believe that God exists. (ie: they don't want to possess probabilistic belief in the existence of God)
So, they possess zero degrees of belief in God.
Lack of belief in God, is atheism.
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