Judaism boasts of no exclusive revelation of eternal truths that are indispensable to salvation—no ‘revealed religion’ in the usual sense of that phrase. Revealed religion is one thing, revealed legislation is another. The voice that let itself be heard on Sinai on that great day did not proclaim
‘I am the Eternal, your God, the necessary, independent being, omnipotent and omniscient, that recompenses men in a future life according to their deeds.’
This is the universal religion of mankind, not Judaism; and the universal religion of mankind, without which men are neither virtuous nor capable of happiness, was not to be revealed there. Actually, it couldn’t have been revealed there, for who would have been convinced of these eternal doctrines of salvation by the voice of thunder and the sound of trumpets? Surely not the unthinking animal-man who hadn’t thought his way through to the existence of an invisible being that governs the visible. The miraculous voice wouldn’t have given him any concepts, so it wouldn’t have convinced him—let alone the sophist, whose ears are buzzing with so many doubts and ruminations that he can’t hear the voice of common sense any more. He demands rational proofs, not miracles. And even if the teacher of religion raised from the dust all the dead who ever trod the earth, in order to establish an eternal truth, the sceptic would say:– Jerusalem: or Religious Power and Judaism (1782), edited by Jonathan Bennett
‘The teacher has awakened many dead, but I don’t know any more about eternal truth than I did before. I do know now that someone can do and say extraordinary things; but there may be several such beings, who aren’t ready to reveal themselves just yet. And all this ·raising-the-dead routine· is so far removed from the infinitely sublime idea of a unique and eternal Deity that rules the entire universe according to its unlimited will, and detects men’s most secret thoughts in order to reward their deeds according to their merits, either here or in the hereafter!Anyone who didn’t already know this, anyone who wasn’t saturated with these truths that are so indispensable to human happiness, and ·therefore· wasn’t prepared to approach the holy mountain, might have been bowled over by the wonderful manifestations but he couldn’t have learned anything from them. – No! All this was presupposed; perhaps it was taught, explained, and placed beyond all doubt by human reasoning during the days of preparation.
The most remarkable part of this passage is the quote from the hypothetical skeptic. Hecht summarizes other aspects of Mendelssohn's position (pp. 362-366). Mendelssohn severed belief from practice and, as Hecht notes, was convinced that Jewish practices could be adhered to without belief. He proved to be correct within a century and a half. However, if you read further in Jerusalem, you should note that this liberalized and rationalized view of religion remains burdened with a fundamental contradiction. It is quite true that Judaism differs from Christianity in that it is more about behavior than belief or salvation. Furthermore, in today's secular society this is the tacit orientation of most American Jews. However, there is none other than an irrational and authoritarian basis for adherence to the Torah or its "legislation". Hence Mendelssohn, though a hero of the Enlightenment (and specifically the Haskalah, or Jewish Enlightenment), is at best a liberal figure, and not as radical as his contemporary Salomon Maimon, not to mention Spinoza. (See also my post on Maimon in my Esperanto blog: Salomon Maimon: filozofo & obstina judo.) Hecht is not concerned with this contradiction, though; she concludes with an apparently laudatory reference to Mendelssohn's pluralism.