Hearing about the Alhambra today it occurred to me that the main innovation of monotheism is certainty.
“There is no God but God” is not a religious statement (“There is a god” would suffice for that), it’s a philosophical claim.
“There is only one god” is not so much a statement of faith in the beyond as it is a statement of faith in what man can know.
It is ironic that this first step towards scientific thinking should be a leap of faith.