system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How Islam spread so quickly after it started?
gpt
* When a bunch of people conquer your town and say you have to pay taxes if you want to worship some other faith, it has a way of making you want to abandon your old faith.

* When that bunch of people say "Hey god said to conquer the huge entrenched empire your town had been a part of for hundreds of years" and then they actually do it, it makes a pretty compelling case that they might be on to something.

As far as how they were able to conquer so much of the Byzantine and Sassanian empires was due to a combination of having good military leaders, luck (the Sassinians had been devastated by a plague shortly before the Muslim conquest), and the fact that the Byzantines and Sassanians had been constantly fighting one another.