For what reasons did Islam spread so quickly in the start?
Islamic cultures and beliefs spread so quickly for two main reasons. The first of these reasons was trade. At the time, Arabic merchants had begun to establish vast trade networks throughout the Middle East, North Africa, East Africa, India, and Asia. With these trade routes came merchants who settled in certain regions and brought their beliefs with them, which led to the steady growth of Islam in these areas.
The second reason behind the rapid spread of Islam was the severe weakness of the Sassanid and Byzantine Empires of the time who had been at war for so long that both empires were dangerously close to collapse. The rise of the Muslim armies and the caliphates led to these empires being almost entirely destroyed due to the speed, support, and momentum of the Islamic armies. This led to Muslim leaders creating empires, or caliphates in the Middle East and North Africa that deeply influenced every aspect of life in these regions. Finally, it is important to note that there was no one central religion in this region at this time and many areas had small localized faiths that could be easily influenced or forced to convert to Islam.