The spread of Islam was extremely rapid, but it causes are not entirely certain as much of the documentation we have from the 7th century is fragmentary, partisan, or otherwise unreliable. One of the major causes was a power vacuum, as the Roman Empire weakened, especially in its control over Egypt and the Sassinad Empire also developed internal weaknesses. The Arabs were more tightly unified, and believed that they had a religious mission to extend their territories. Because they treated conquered peoples quite generously, allowing, for example, more freedom to Christian heretics than the Byzantine Empire, they were able to consolidate their conquests with often minimal threat of uprisings or resistance from the conquered peoples.
Meanwhile, the Islam usually spread first in the islamic country such as; Middle east, Pakistan, Syria and many more. For the Muslim they usually believe that God is only one and they called Him Allah (exclusively for them). They also believe the allah sees all things and He is present everywhere.