This criticism levelled at Islam by theologians shows the correctness of the assertion that the appreciation of the truth of a religion largely depends on the mental, subjective attitude one holds towards the subject. It proves the inadequate understanding of the theologian who approaches a subject only from the outside and can no more refute the truth of a religion than he can prove it. And if the scholar is Muslim, then his presentation proceeds from his inner religious conviction which is not accessible to Christian theologians. Someone who believes that the message of Muhammad was a revelation from God no longer wonders about the origin of the message he proclaimed. Similarities with the ideological world of other religions do not worry him in the least. They are-- the revealed Truth being taken as the criterion-- the remains of former religious truths, the authority of which is being reaffirmed in the new revelation. From this angle, similarities between the religious world of Islam and that of Ebionite or Nestorian Christianity are explained by the fact that it was precisely in those Christian fractional groups, which had been accused of heresy by the High Church, that religious truths from which orthodox teaching had digressed were preserved. It is not Church orthodoxy, therefore, which is the criterion by which Islam is to be judged, but rather the other way round. If, on the other hand, the scholar does not believe in Islam, and does not accept the fact that God has revealed himself through Muhammad or that divine revelation is at all possible, Islam cannot but appear to him as an eclectic conglomeration of heterogeneous ideas and teachings, as a syncretism. Then, of course, to investigate the origins of the borrowed ideas and to prove the lack of originality of the Prophet appears as an exciting task to the scholar (146:1)

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The Light Shineth in Darkness
Udo Schaefer