Mendelssohn later had reason to thank Lavater for having through thoughtlessness or pious cunning drawn him out of his diffidence and seclusion. Mendelssohn had indeed expressed his relations to Judaism and his co-religionists so vaguely that onlookers might have been misled. In public life he was a philosopher and an elegant writer, who represented the principles of humanity and good taste, and apparently did not trouble about his race. In the darkness of the Ghetto he was a strictly orthodox Jew, who, apparently unconcerned about the laws of beauty, joined in the observance of every pious custom. Self-contained and steadfast though he was in reality, he seemed to be a twofold personality, revealing the one or the other as he was in Christian or in Jewish society. He could not stand up in defense of Judaism without, on the one hand, affronting Christianity by his philosophical convictions, and, on the other, showing, if ever so lightly, his dissatisfaction with the chaotic traditions of the synagogue, and so offending the sensibility of his co-religionists and quarreling with them. Neither of these courses, owing to his peace-loving character, entered his mind. He would have been able to pass his life in an attitude of silence, if Lavater's rude importunity had not dragged him out of this false position, altogether unworthy of a man with a mission. Painful as it was to reveal his innermost thoughts upon Judaism and Christianity, he could not hold his peace at this challenge, without being considered a coward even by his friends. These reflections weighed heavily upon him, and caused him to take up the glove.
He skillfully carried on the contest thus forced upon him, and was ultimately victorious. At the end of 1769, in a public letter addressed to Christendom and Lavater, its representative, Mendelssohn in the mildest form wrote most cutting truths, whose utterance in former times would inevitably have led to bloodshed or the stake. Mendelssohn had examined his religion since the days of his youth, and found it true. Philosophy and belles-lettres had with him never been an end, but the means to prepare him for testing Judaism. He could not possibly expect advantage from adherence to it; and as for pleasure—
"O my worthy friend, the position assigned to my co-religionists in civil life is so far removed from all free exercise of spiritual powers, that one's satisfaction is not increased by learning the true rights of man. He who knows the state in which we now are, and has a humane heart, will understand more than I can express."
If the examination of Judaism had not produced results favorable to it, what would have chained him to a religion so intensely and universally despised, what could have prevented him from leaving it? Fear of his co-religionists, forsooth? Their secular power was too insignificant to do any harm.
"I do not deny that I have noticed in my religion certain human additions and abuses, such as every religion accepts in course of time, which unfortunately dim its splendor. But of the essentials of my faith I am so firmly and indisputably assured, that I call God to witness that I will adhere to my fundamental creed as long as my soul does not assume another nature."
He was as opposed to Christianity as ever, for the reason which he had communicated to Lavater verbally, and which the latter should not have concealed, namely, that its founder had declared himself to be God.
"Yet, for my part, Judaism might have been utterly crushed in every polemical text-book, and triumphantly arraigned in every school composition, without my ever entering into a controversy about it. Without the slightest contradiction from me, any scholar or any sciolist in subjects Rabbinic might have constructed for himself and his readers the most ridiculous view of Judaism out of worthless books which no rational Jew reads, or knows of. The contemptible opinion held of Jews I would desire to shame by virtue, not by controversy. My religion, my philosophy, and my status in civil life are the weightiest arguments for avoiding all religious discussion, and for treating in public writings of truths equally important to all religions."
Judaism was binding only upon the congregation of Jacob. It desired proselytes so little, that the rabbis had ordained that any person who offered to unite himself to this religion was to be dissuaded from his design.
"The religion of my fathers does not care to be spread abroad; we are not to send missions to the two Indies or to Greenland, to preach our belief to remote nations. I have the good fortune to possess as friends many excellent men not of my creed. We love each other dearly, and never have I said in my heart, 'What a pity for that beautiful soul!' It is possible for me to recognize national prejudices and erroneous religious opinions among my fellow-citizens, and nevertheless feel constrained to remain silent, if these errors do not directly affect natural religion or natural law (morality), but are only accidentally connected with the advancement of good. It is true that the morality of our actions does not deserve the name, if based upon error.... But as long as truth is not recognized, as long as it does not become national, so as to work as powerful an effect upon the great mass of the people as ingrained prejudice, the latter must be almost sacred to every friend of virtue. These are the reasons that religion and philosophy give me to shun religious disputes."