What caused the spread of monotheism?

Did the switch to monotheism, first by Christians and then by Moslems represent a major change in thought? It would seem to. The idea of a universe governed by scientific law is much more compatible with the idea of a single god than with a group of them, even if the early church did try to suppress science. What could have been the cause of the switch? The Roman Empire and Middle Ages do not seem in any way suggestive of a major change in thought, yet the scientists of the Renaissance and early Enlightenment were deeply religious. Galileo, for all his troubles with the Church, never abandoned his faith. At a young age Pascal gave up a fruitful career as a mathematician to pursue theology. Newton devoted the final years of his life to religion. Science flourished in the early days of Islam, though the golden age did not last very long. Was there something going on, or was it just by chance that monotheism took hold?