Here is the proof:
(1) If God exists, God exists necessarily.
(2) An agnostic atheist does not claim to know that God does not exist.
(3) Claiming to not know that God does not exist entails believing that God’s existence is possible.
(4) Therefore the agnostic atheist believes that it is possible that God necessarily exists.
(5) If it is possibly necessarily true God exists, God necessarily exists.
(6) The agnostic atheist believes God exists.
I’m not going to share the issues, maybe it can be a fun puzzle.