In Turtledove's "Through Darkest Europe", two theologians change opinions regarding religion and reason.
During the 11th century Al-Ghazali, who in the real world argued that Islamic faith was superior to philosophy and science, taught the two were compatible. And later in the 13th century Thomas Aquinas, who in the real world sought to reconcile religion and reason, taught that Christianity and reason were incompatible, and that religious faith should override reason.
As a consequence of this shift in theology, North Africa and Middle East now form the prosperous, democratic and progressive First World, while Europe is an impoverished hot spot of christian extremism, tribalism and terrorism.
But what real changes, if any, would there have been if Al-Ghazali and Thomas Aquinas had taught opposite of what they did in OTL regarding the compatibility of religion and reason?
Alternatively, if the change of opinions seems implausible, what if Al-Ghazali and Thomas Aquinas didn't exist, and instead we had another Islamic theologian teaching compatibility of faith with philosophy and science, and another Christian theologian teaching that faith is superior to reason?