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Inasmuch as the spiritual value of man's faith depends on its being an outcome of free choice and not of compulsion, the visible or audible appearance of a "message from the skies" would, by its very obviousness, nullify the element of free choice and, therefore, deprive man's faith in that message of all its moral significance.
If it had been Allah's Will and Plan to force people's will, He could quite easily have forced the Makkans. But His Will and Plan work differently. His revelation is meant to train man's own will so that it conforms to Allah's beneficent purpose.