I may not agree with all of the Saadawi's ideas but I liked her grandma's story as the concept of God is inseparable from concept of "Justice" and that is why, believing in the "Judgment Day" is the core concept of religions and easy to grasp for all individuals irrespective of their literacy levels. The problem arises when some people start emphasizing the need for certification in understanding and believing in God. Surely, then God becomes an unreachable concept that needs mediation of "specialists in God", a commodity that not everyone can afford.
If we turn to specialists (Philosophers) , we will hear an argument as following,
There can be two scenarios when we look at "what is right" in respect with the concept of God;
A; "Something is right because God says, it is right"
This concept is against concept of justice as it is based on the concept of "Might is Right" and this concept has been used to justify all sorts of crimes from Genocides, discrimination and suppression of minorities, women, children and authoritarian rules.
B; "God says, something is right because it is right"
This concept is based on the concept of Justice and we don't need all the times, the specialists to tell us what is right as our reasoning and conscience can guide us to what is right. Reason is the best mediator between the Creator and the subjects.
Personally, I believe that, there are billions of people who believe in God (and it makes it an important subject particularly that, it is exploited for political reasons and denials can't do any good) according to their capacities and it is natural that their concepts do not match with each other. Humans err and that is why they need the concept of God to take refuge and if humans had flawless brains, they either wouldn't need this concept at all or all were perfect faithfuls. Modern societies have solved this problem by giving the people "the rights to be wrong as long as it is not harming others" and that is closer to justice than forcing people to follow particular set of concepts, that a lot of people find them conflicting with their reasoning or conscience.