Final answer:
The success of Muhammad and his followers in spreading Islam can be attributed to various reasons, including the promise of eternal reward, the moral and ethical code of Islam, the ease of adhering to the Five Pillars of Faith, and the doctrine of salvation preached by Muhammad.
Step-by-step explanation:
The option that was not a reason for the success of Muhammad and his followers in spreading Islam is The faith was simple to explain and to understand. The other options were all factors that contributed to the spread of Islam. Believers were attracted by the promise of eternal reward, the moral and ethical code of Islam, the ease of adhering to the Five Pillars of Faith compared to the Ten Commandments, and the doctrine of salvation preached by Muhammad.
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