Who or what dictated the Koran to Mohammad in that cave? Or did Mo just make it all up?
I suspect a combination of demonic influence, delusion, genuine though perhaps misguided belief in his mission and opportunism.
Much like Joseph Smith, probably.
That I do not believe Mohammed (or Joe) to be genuine prophets of God does not necessarily mean that all who follow them today are therefore worshiping demons, IMO.
People who were indisputably Christian believers have over the centuries followed men who led them into committing great atrocities, quite frequently in the name of Christ. Does not mean that they were therefore no longer worshipers of God, only that they were misled and at least for that time no longer walking in his light.
Islam is in some ways the greatest of all Christian heresies.