There is a difference in the middle ages from the fall of Rome until around the high middle ages. [Aquinas Rambam, Ibn Rushd.] Until then people were going with the neo platonic philosopher. Plotinus. so in understanding the unity of God, they were going with the idea of emanation --or an overflowing of God's light. but at no point were they saying that only God exists. Rather they understood there is a difference between the Creator and the created. But the problems of reconciling pure Monotheism [Divine Simplicity] with Neo-Platonic thought, led to abandoning Plotinus and going with Aristotle. Muslims had been going with Aristotle for a long time. But Jews and Christians had been trying for  to go with Plotinus. At some point. they gave up and decided that was not going to work. So you get the Rambam going a lot more with Aristotle than his predecessors. [But not completely. He still retains a lot of Neo Platonic thought.] Then finally, Aquinas made the final jump to Aristotle. 

But as Kelley Ross noted, that jump might have landed everyone in more problems than they started out with. When those problems became apparent. it would have made sense to rethink things and return to Plato. [taking kant into accountin understanding plato]

[I should add that not all Rishonim were on board with either philosopher. The one that comes tomind in Nahmanidess. And it is his approach  that is the reason the religious world is against all philosophy [as David Bronson pointed out to me.]