I think it is them who should prove that Allah does not exist!
if not, then who created the heavens and earth and what is between them including us? is it existed without a causer. is there is something made without a maker? creatures/creation(coming from no-existence to the existence/being) without creator? a drawing without an artist?
If they just believe what the eyes sees without the brain then they will see the drawing without the artist, the music without the musician, the body without the spirit, the day without the sun, the machine without its power, the plan without the planner...etc. and they should know that the eyes of the heart/brain sees better than they eyes of the face.. the eyes of the face is just like a spy or sensor to the eyes of the brain. as the eyes of the face will not see the most close thing to her(as it will not see the nose or ears) but the brain will realize it.. the eyes of the face will see the mirage and think it is something while it is nothing but the brain/eyes will realize it.. the eyes of the face will be cheated when seeing the reflection of the moon on the surface of the river while the brain will realize it.. the eyes of the face will see the big small and small big and the moving stable and the stable moving but only with the brain we will realize its truth.
We are made as a category above the animals because of that brain(6th sense)(king of our kingdom).