It is important to discuss the doctrine of Islam and stay away from discussing Muslims unless that Muslim is Mohammed. He is the only Muslim who really matters. Islam is what Mohammed (and Allah) says it is, not what a Muslim says it is.
However, it is important to understand that the doctrine can only impact the Kafir when it is followed by Muslims. If the doctrine was never practiced, the Kafir would not be affected.
The relationship of the Muslim to doctrine is important. The more devout the Muslim, the closer they adhere to the doctrine. Since Islam is dualistic, with two contradictory sets of principles, the Muslim can choose, at any time, which set to follow.
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