I will quote in this connection the words of the wisest of all men, King Solomon...

Excerpt of a letter, the date of which was not available

I trust you will not take it amiss if I will quote in this connection the words of the wisest of all men, King Solomon, "G-d made man straight, but they sought many accounts." In other words, man often confuses himself with delving, unneces-sarily, into inquiries and accounts of things which should be taken for granted and which do not really present any problems. Needless to say, that the more intellectual a person is, the more he is inclined to seek "accounts" and, consequently, the more apt he is to get confused.
This reminds me of the episode which a professor of medicine once told me. On one occasion when he was learning anatomy, and particularly the anatomy of the leg, describing the various muscles, etc., amounting to hundreds, all of which are so perfectly coordinated in the motion of the leg during walking, he became so engrossed in the details (all the more so being a man of great intellect) that momentarily he found his walking difficult and quite complicated as he began to analyze the working of each muscle and joint, etc. The moral is obvious.
Now to your question:
I will first briefly state here the logical basis of the Truth that the Torah and Mitzvos [commandments] have been given to us Jews by Divine Revelation. This is not very difficult to prove, since the proof is the same as all other evidence that we have of historic events in past generations, only much more forcefully and convincingly.
By way of illustration: If you are asked, how do you know there existed such a person as Maimonides (whom you mention in your letter) author of Yad HaChazakah, Sefer HaMitzvos, etc., you will surely reply that you are certain about his existence from the books he has written, and although Rambam (Maimonides) lived some 800 years ago, his works now in print have been reprinted from earlier editions, and those from earlier ones, still uninterruptedly, going back to the very manuscript which the Rambam wrote in his own hand. This is considered sufficient proof even in the face of discrepancies or contradictions from one book of Rambam to another. Such contradictions do not demolish the above proof, but efforts are made to reconcile them, in the certainty that both have been written by the same author.
The same kind of proof substantiates any kind of historic past, which we ourselves have not witnessed, and all normal people accept them without question, except those who for some reason are interested in falsification.
In many cases the authenticity of an historic event is based on the evidence of a limited group of people. Even where there is room to suspect that the witnesses were perhaps not quite disinterested, if there is nothing to compel us to be suspicious (and especially if we can check the evidence and countercheck it) it is accepted as fact.
Now suppose that 600,000 parents would today say to their children, "This morning you and we were all gathered at a certain place, and we all heard a Heavenly voice proclaim the Decalogue." The children would not accept this for they would say: "If we were there with you, why did we not hear or see anything?" Now, making the single assumption that human reactions have not essentially changed in the course of centuries, I assume that such would have been the reaction also in the previous century, and two centuries ago and so on, until we reach the gener-ation whose parents witnessed the event of the Giving of the Torah on Mount Sinai.
And let it be emphasized again that during this long chain of tradition, there has been no break, nor has the number of transmitters at any time been reduced to less than many hundreds of thousands, for at no time was there less than one million Jews in the world, Jews from all walks of life, who had no personal ax to grind, etc., yet in each generation of the uninterrupted and unbroken history of our people, this event was accepted as authentic history and the text of the Decalogue remained exactly the same. This is certainly undeniable evidence according to all the rules of scientific proof accepted today.
The same cannot be said of any other religions in the world, which you mentioned, such as Buddhism, Christianity, Islam. In the case of all these religions, there is a definite break, or the tradition narrows down to a single person such as Buddha, Mohammed, or the founder of Christianity, who transmitted his teachings to a group of 12 Apostles.