In the same year (October, 1763), he received a distinction from King Frederick, characteristic of the low condition of the Jews in Prussia. This honor was the privilege of being a protected Jew (Schutz-Jude), i. e., the assurance that he would not some fine day be expelled from Berlin. Hitherto, he had been tolerated in Berlin only as a retainer of his employer. The philosophical King Frederick sympathized with the antipathy of his illustrious enemy Maria Theresa to the Jews, and issued anti-Jewish laws worthy of the Middle Ages rather than of the eighteenth century, so boastful of its humanity. He wished to see the Jews of his dominions diminished in number, rather than increased. Frederick's "general privilege" for the Jews was an insult to the age. Marquis d'Argent, one of Frederick's French courtiers, who in his naïveté could not conceive that a wise and learned man like Mendelssohn might any day become liable to be driven out of Berlin by the brutal police, urged Mendelssohn to sue for the privilege of protection, and the king to grant it. However, a long time elapsed before the dry official document granting it reached him. At last Mendelssohn became a Prussian "Schutz-Jude."

The philosophical "Schutz-Jude" of Berlin now won great success with a work, which met with almost rapturous admiration from his contemporaries in all classes of society. Two decades later this production was already obsolete, and at the present day has only literary value. Nevertheless, when it appeared, it justly attained great importance. Mendelssohn had hit upon the exact moment to bring it forward, and he became one of the celebrities of the eighteenth century. For almost sixteen centuries Christianity had educated the nations of Europe, governed them, and almost surfeited them with belief in the supernatural. It had employed all available means to effect its ends, and finally, when the thinkers awakened from their slumber induced by its lullabies, to inquire into the certainty secured by this announcement of salvation which promised so much, serious people said with regret—whilst sceptics chuckled with brutal delight—that it offered delusive fancies in the place of truth.

In serious compositions, or in satires, the French thinkers of the eighteenth century—the whole body of Materialists—had revealed the hollowness of the doctrine, in which the so-called civilized peoples had found comfort and tranquillity for many centuries. The world was deprived of a God, the heavens were enshrouded in mist; all that had hitherto seemed firm and incapable of being displaced was turned topsy-turvy. The doctrine of Jesus had lost its power of attraction, and become degraded in the eyes of the earnest and thoughtful to the level of childish fables. Infidelity had become a fashion. With the undeifying of Jesus appeared to go hand-in-hand the dethronement of God, and doubt of the important dogma of the immortality of the soul, which Christian theology had borrowed from Greek philosophy and, as always, adorning itself with strange feathers, had claimed as its original creation. Thereupon depended not merely the confidence of mankind in a future existence, but also the practical morals of the present. If the soul is mortal and transient, they thought in the eighteenth century, then the acts of man are of no consequence! Whether he be good or evil, virtuous or criminal, on the other side of the grave there was no retribution. Thus, after the long dream of many centuries, the civilized portion of mankind again fell into the despondency prevalent in the Roman society of the empire; they were without God, without support, without moral freedom, without stimulus to a virtuous life. Man had been degraded to a complicated machine.

Mendelssohn was also biased by the prejudice that the dignity of man stands and falls with the question of the immortality of the soul. He therefore undertook to restore this belief to the cultured world, to discover again the lost truth, to establish it so firmly and ward off materialistic attacks upon it so decisively, that the dying man should calmly look forward to a blissful future and to felicity in the after-life. He composed a dialogue called "Phædon, or the Immortality of the Soul." It was to be a popular book, a new doctrine of salvation for the unbelieving or sceptical world. Therefore he gave to his dialogue an easily comprehensible, attractive style, after the pattern of Plato's dialogue of the same name, from which he copied also the external form. But Plato supplied him with the mere form. Mendelssohn caused his Socrates to give utterance to the philosophy of the eighteenth century through the mouth of his pupil, Phædon.

His starting-point, in proving the doctrine of the immortality of the soul, is the fact of the existence of God, of which he has the highest possible certainty. The soul is the work of God, just as the body is; the body does not actually perish after dissolution, but is transformed into other elements; much less, then, can the soul, a simple essence, be decomposed, and perish. Further, God has acquainted the soul with the idea of immortality, has implanted it in the soul. Can He, the Benevolent and True One, practice deception?

"If our soul were mortal, then reason would be a dream, which Jupiter has sent us that we may forget our misery; and we would be created like the beasts, only to seek food and die."

Every thought inborn in man must for that reason be true and real.

In demonstrating the doctrine of immortality, Mendelssohn had another noble purpose in view. He thought to counteract the malady of talented youths of the day, the Jerusalem-Werthers, who, without a goal for their endeavors, excluded from political and elevating public activity, lost in whimsical sentimentality and self-created pain, sank to thoughts of suicide, which they carried out, unless courage, too, was sicklied over. Mendelssohn, therefore, in his "Phædon" sought to inculcate the conviction, that man, with his immortal soul, is a possession of God, and has no manner of right to decide arbitrarily about himself and his life, or about the separation of his soul from his body—feeble argumentation, but sufficient for that weakly, effeminate generation.

With his "Phædon," Mendelssohn attained more than he had intended and expected, viz., "conviction of the heart, warmth of feeling," in favor of the doctrine of immortality. "Phædon" was the most popular book of its time, and was perused with heart and soul. In two years it ran through three editions, and was immediately translated into all the European languages, also into Hebrew. Theologians, philosophers, artists, poets, such as Herder, Gleim, and Goethe, then but a youth, statesmen, and princes—men and women—were edified by it, reanimated their depressed religious courage, and, with an enthusiasm which would to-day appear absurd, thanked the Jewish sage who had restored to them that comfort which Christianity no longer afforded. The deliverance by Mendelssohn, the Jew, was as joyfully welcomed by the world grown pagan, as in an earlier epoch that effected by the Jews, Jesus and Paul of Tarsus, was welcomed by the heathens. His contemporaries were delighted both with the contents and the form, with the glowing, fresh, vigorous style, a happy, artistic imitation of Plato's dialogues. From all sides letters of congratulation poured in upon the modest author. Everyone of the literary guild who passed through Berlin eagerly sought out the Jewish Plato, as one of the greatest celebrities of the Prussian capital, to have a word with him. The Duke of Brunswick seriously thought of securing the services of Mendelssohn for his state. The Prince of Lippe-Schaumburg treated him as a bosom friend. The Berlin Academy of Sciences proposed him as a member. But King Frederick struck the name of Mendelssohn off the list, because, as it was said, he desired at the same time to have the Empress Catherine admitted into the learned body, and she would be insulted in having a Jew as a companion. Two Benedictine friars—one from the convent of Peter, near Erfurt, the other from the convent of La Trappe—addressed Mendelssohn, the Jew, as the adviser of their conscience, for instruction in moral and philosophical conduct. The book "Phædon," out of date in twenty years, as remarked above, raised its author to the height of fame. He was fortunate, because he introduced it to the world exactly at the right moment.

An incident vexatious in itself served to exalt Mendelssohn to an extraordinary degree in the eyes of his contemporaries, and to invest him with the halo of martyrdom. John Caspar Lavater, an evangelical minister of Zürich, an enthusiast who afterwards joined the Jesuits, thought that he had found in Mendelssohn's intellectual countenance a confirmation of his deceptive art, the reading of the character and talents of a man from his features. Lavater asserted that in every line of Mendelssohn's face the unprejudiced could at once recognize the soul of Socrates. He was completely enchanted with Mendelssohn's head, raved about it, desiring to possess a well-executed model, in order to bring honor upon his art. Mendelssohn having caused his Phædon to speak in so Greek a fashion that no one could have recognized the author as a Jew, Lavater arrived at the fantastic conclusion that Mendelssohn had become entirely estranged from his religion. Lavater had learned that certain Berlin Jews were indifferent to Judaism, and forthwith reckoned Mendelssohn amongst their number. There was the additional fact that, in a conversation reluctantly entered upon with Lavater, Mendelssohn had pronounced calm, sober judgment upon Christianity, and had spoken with a certain respect of Jesus, though with the reservation, "if Jesus of Nazareth had desired to be considered only a virtuous man." This expression appeared to Lavater the dawning of grace and belief. What if this great man, this incarnation of wisdom, who had become indifferent towards Judaism, could be won over to Christianity! This was the train of thought which arose in Lavater's mind after reading "Phædon." Ingenuous or cunning, he spread his net for Mendelssohn, and thus showed how ignorant he was of his true character. About this time, a Geneva professor, Caspar Bonnet, had written in French a weak apology, entitled "Investigation into the Evidences of Christianity against Unbelievers." This work Lavater translated into German, and sent to Mendelssohn, with an awkward dedication, which looked like a snare (September 4, 1769). Lavater solemnly adjured him to refute publicly Bonnet's proofs of Christianity, or, if he found them correct, to do "what sagacity, love of truth, and honesty would naturally dictate, what a Socrates would have done, if he had read this treatise, and found it unanswerable." If Lavater had been really acquainted with the secrets of the heart, as he prided himself, he would have understood that, even if Mendelssohn had severed all connection with Judaism, Christianity was still more repugnant to him, and that sagacity, that is to say, regard to profit and the advantages of a pleasant existence, was altogether lacking in his character. Lavater did not desire to expose him before the public, but he wished to create a sensation, without thinking what pain he was causing the shy scholar of Berlin.