People who deny the efficacy of any rationalistic investigation into religion seem to me somewhat to be contradicting themselves. For instance, the Christian claims that his religion is the only true one, because it was revealed to so-and-so. The Mohammedan makes the same claim for his religion; his is the only true one, because it was revealed to so-and-so. But the Christian says to the Mohammedan, “Certain parts of your ethics do not seem to be right. For instance, your books say, my Mohammedan friend, that an infidel may be converted to the religion of Mohammed by force, and if he will not accept the Mohammedan religion he may be killed; and any Mohammedan who kills such an infidel will get a sure entry into heaven, whatever may have been his sins or misdeeds.” The Mohammedan will retort by saying, “It is right for me to do so, because my book enjoins it. It will be wrong on my part not to do so.” The Christian says, “But my book does not say so.” The Mohammedan replies, “I do not know; I am not bound by the authority of your book; my book says, ‘Kill all the infidels’. How do you know which is right and which is wrong? Surely what is written in my book is right and what your book says, ‘Do not kill,’ is wrong. You also say the same thing, my Christian friend; you say that what Jehovah
declared to the Jews is right to do, and what he forbade them to do is wrong. So say I, Allah declared in my book that certain things should be done, and that certain things should not be done, and that is all the test of right and wrong.” In spite of that the Christian is not satisfied; he insists on a comparison of the morality of the Sermon on the Mount with the morality of the Koran. How is this to be decided? Certainly not by the books, because the books, fighting between themselves, cannot be the judges. Decidedly then we have to admit that there is something more universal than these books, something higher than all the ethical codes that are in the world, something which can judge between the strength of inspirations of different nations. Whether we declare it boldly, clearly, or not — it is evident that here we appeal to reason.
Now, the question arises if this light of reason is able to judge between inspiration and inspiration, and if this light can uphold its standard when the quarrel is between prophet and prophet, if it has the power of understanding anything whatsoever of religion. If it has not, nothing can determine the hopeless fight of books and prophets which has been going on through ages; for it means that all religions are mere lies, hopelessly contradictory, without any constant idea of ethics. The proof of religion depends on the truth of the constitution of man, and not on any books. These books are the outgoings, the effects of man’s constitution; man made these books. We are yet to see the books that made man.
From Volume 1 : Complete Works of Swami Vivekananda : Reason and Religion