"Islam is a religion that is essentially rationalistic in the widest sense of this term considered etymologically and historically. The definition of rationalism as a system that bases religious belief on principles furnished by the reason applies to it exactly... It cannot be denied that many doctrines and systems of theology and also many superstitions, from the worship of saints to the use of rosaries and amulets, have become grafted on the main trunk of Muslim creed. But in spite of the rich development, in every sense of the term, of the teachings of the prophet, the Quran has invariably kept its place as the fundamental starting point, and the dogma of unity of God has always been proclaimed therein with a grandeur, a majesty, an invariable purity and with a note of sure conviction, which it is hard to find surpassed outside the pale of Islam.
This fidelity to the fundamental dogma of the religion, the elemental simplicity of the formula in which it is enunciated, the proof that it gains from the fervid conviction of the missionaries who propagate it, are so many causes to explain the success of Mohammedan missionary efforts. A creed so precise, so stripped of all theological complexities and consequently so accessible to the ordinary understanding might be expected to possess and does indeed possess a marvelous power of winning its way into the consciences of men." ("La Propaganda Chretienne it Adversaries Musulmans", Paris, 1890, quoted by T.W. Arnold in The Preaching of Islam, London, 1913).