Maimuni's commentary on the Mishna arose out of the author's mental organization, which ever strove for clearness, method and symmetry. It was the first scientific treatment of the Talmud, and only so clear and systematic a thinker as Maimuni could have originated it, for the construction of the Talmud seems to be directly opposed to an orderly arrangement. The luminous introductions to the several parts of the commentary especially give evidence of its scientific character. In them he reveals complete command over the material, as well as a logical conception of the method to be pursued.

Maimuni treated, with special predilection, those points of the Mishna which have a scientific coloring, and into the treatment of which the principles of mathematics, astronomy, physics, anatomy, ethics and philosophy could be introduced. Here he was in his element. In such parts he could show that the doctors of the Mishna, the upholders of tradition, knew science also, and based their works upon it. Especially did he aim at establishing that the Mishna contains a sound ethical and a deep philosophical conception of God. To this end he turned his attention with particular interest and thoroughness to the Agadic elements in the Mishna, which till then had been little or only occasionally noticed. He further explained the nature of tradition, maintaining that not all that is contained in the Mishna is tradition. For a traditional doctrine must be positive, and ought not to be open to doubt or uncertainty. Unconsciously Maimuni by this theory put himself in opposition to the Talmud, and undermined its firm position.

The tractate of the Mishna, which combines, like a string of pearls, the sayings of the fathers (Aboth), appeared in the eyes of Maimuni a veritable treasure-trove. In explaining these he could display the whole wealth of his world of thought, and he thus saturated Talmudical Judaism with philosophical ideas. But he thereby became the victim of self-delusion. It was important for the future that Maimuni, in his unconscious self-deception, undertook for the first time to develop a Jewish system of belief. Since Judaism, according to his views, was nothing more than revealed philosophy, it ought to dominate the beliefs and opinions of men as well as their religious and moral conduct; ay, the one more than the other, as morality has no value in itself, and is only the fruit of right knowledge. He, accordingly, assumed as certain and positive that Judaism defines for us not only what we must do, but what we must believe; that it asserts certain ideas as irrefragable truth. Maimonides drew up thirteen of such doctrines or articles of belief:—The belief in the existence of God; in His indivisible unity; in His incorporeality and insusceptibility of change; in His eternity and existence before the world; in His absolute claim to our adoration (Monotheism); in the prophetic inspiration of chosen men; in Moses as the greatest prophet, with whom no other prophet can be compared; in the divinity of the Torah; in its unalterability; in God's providence; in His just reward and punishment; in the future appearance of the Messiah; and, finally, in the resurrection of the dead. Although these articles of faith rest on investigation, and therefore cannot claim unquestioning acceptance, yet, according to Maimuni, no one can be considered a true Israelite or Jew who does not acknowledge them all as true; he who denies a single one of them is a heretic (Min, Epicoros), he does not belong to the community of Judaism, and cuts himself off from the hope of future bliss.

Maimuni thus, on the one hand, raised the Jewish creed to the height of rational knowledge, and, on the other, set bounds to the free development of thought. Hitherto religious action only was valued as the characteristic of Jewish life. Maimuni now called a halt to free thought, marked the boundary line between belief and heresy, not in the firm province of religious practice, but in the shifting ground of religious belief, and brought the ethereal element of thought under rigid formulæ.

Great as the work of Maimuni in his commentary on the Mishna undoubtedly is, although he applied to it infinite learning, wealth of intellect, and systematic arrangement, yet he did not obtain a reputation corresponding to its merit. The reason of this was that among the Jews of Egypt and the East, to whom the work, being in Arabic, was most of all accessible, there was but the faintest appreciation of scientific treatment. The great work was at first scarcely noticed in the East. His pupils, to whom he gave lectures on the same plan, and who revered him as the incarnation of wisdom, spread his reputation abroad. One of his earliest disciples, Solomon Kohen, who traveled to southern Arabia (Yemen), was full of his praise, and impressed on the congregation there that, in time of need, they should apply to Maimuni for consolation and support.

In Egypt far-reaching changes had crept in, which produced a favorable turn in the fortunes of the Jews of that empire and the neighboring countries. The Fatimide Caliph died, or was deposed, and the great Saladin, the model of royal magnanimity and chivalry in that barbarous age, succeeded to the government (September, 1171). At first the celebrated Ajubide only held the office of Vice-Field-Marshal of Nureddin; gradually he acquired absolute supremacy over Egypt and a part of Palestine, Syria, and even the districts about the Euphrates, and the Caliphate of Bagdad obeyed his rule. His empire became a safe asylum to the oppressed Jews. Saladin was just to the Jews, as indeed towards every one, even his bitterest enemies. Under him the Jews rose to great prosperity and distinction.

At first the fall of the Fatimide Caliphate, and the subjection of the surrounding countries belonging to it, under the Abbasid or Sunnite Caliphs of Bagdad, set loose fanaticism which was felt by the Jewish congregations of Yemen. In that place two Shiites had seized upon the government, and they compelled the Jews to embrace Islam under threat of great suffering. Here also, as in Africa and southern Spain, the Jews outwardly pretended to adopt the Mahometan religion (about 1172). But as the grossest ignorance prevailed among them, there was danger that the unthinking multitude would proceed from pretence to reality, and fall away from Judaism altogether. This fear became real when a Jewish apostate preached to the congregation that Mahomet is mentioned in the Torah, and that Islam was a new, divinely announced revelation, which was intended to supersede Judaism. In addition, at just about this time, there appeared a Jewish enthusiast in Yemen, who proclaimed himself to be the forerunner of the Messiah, endeavored to instil in the Jews the belief that their affliction was the harbinger of the speedy approach of the Messianic empire, and bade them hold themselves in readiness for that event, and divide their property with the poor. This enthusiastic hope, to which many clung as drowning men to a straw, threatened to bring the direst misfortune on the heads of the Yemen Jews. The pious abandoned themselves to despair in the contemplation of these proceedings, altogether lost their heads, and knew not what plan they should adopt. At this point, Jacob Alfayumi, the most learned and most respected man among them, turned to Maimuni, of whom he had heard through his disciples, for counsel and consolation, described to him their sufferings and apprehensions, and begged him to send a reply.

Maimuni accordingly sent a letter of consolation, in Arabic, to the congregation of Yemen, directed personally to his correspondent, but having reference to all the members (Iggeret Teman). In spite of its small compass, it contains valuable matter, and bears witness to the writer's lofty soul and spiritual refinement. He sought in it to elevate the sufferers to the height of spiritual consciousness, on which suffering for religion's sake loses its sting, and darkness appears as the inevitable antecedent of the break of day. He expressed himself on the relation of Judaism to Christianity and Islam with an acuteness and precision which reflect his profound conviction. It was certainly sad to reflect, remarks the sage of Cairo, that there should have occurred cruel persecutions of the Jews in two opposite directions; in the West by the Almohades, and in the East by the Mahometans of Yemen. Nevertheless they were not unexpected, for the prophets had announced them quite distinctly. "Because God has specially distinguished us, sons of Israel, through His grace, and has appointed us the upholders of the true religion and the true creed, the nations hate us, not only on our own account, but on account of the divinity which lives in our midst, in order to thwart in some measure the divine will." Since the revelation on Sinai there had never been a time when Judaism and its professors had not been exposed to sufferings and persecutions. The nations had manifested their hate in three different forms; either with the sword, like Amalek, Sennacherib, Nebuchadnezzar, Titus, and Hadrian, in order utterly to root out from the earth the nation that possessed the truth; or with the false tricks of sophistical persuasion, like the Persians, Greeks, and Romans, with a view to refute and falsify the doctrines of Judaism; or finally under the mask of revelation, as it were, in the garb of Judaism, in order to juggle it out of existence. The principle inimical to Judaism had at length discovered that it was unable to annihilate the upholders of God's religion, or to tear it out of their hearts; and now it hoped to destroy them by a crafty device. It pretended also to have received a revelation acknowledging that on Sinai to have been authorized for a time, but declared that it now had no further validity. This hostile principle, which sought the banishment of the divine from earth, attempted to substitute a stuffed figure for a godly child, and falsify Judaism. The new revelations of Nazareth and Mecca, compared with Judaism, were like well-executed statues of a man, compared with a real man full of life and energy. All this bitter enmity of the nations of the earth against Israel and its divine religion had been foreseen by the prophets, especially by Daniel, who at the same time foretold the victory of Judaism over superstition. "And now, brethren," so Maimuni addressed the congregation of Yemen in his letter, "consider well these truths, and do not let yourselves be discouraged by the superabundance of your woe. Its purpose is to test you, and to show that the posterity of Jacob, the descendants of those who received the Law on Sinai, are in possession of the true Law." Furthermore, he pointed out that it was wrong to calculate the Messianic period, as the Yemen enthusiast thought he had succeeded in doing; for it can never be exactly determined, it having been purposely concealed as a deep secret by the prophets.

Lastly, Maimuni exhorted Jacob Alfayumi to circulate his letter among the congregations of Yemen, that it might strengthen them in their faith, but to take great precautions when reading it that no traitor might be given the opportunity of making it the pretext for an accusation. He himself, said Maimuni, wrote in anxiety as to the evil consequences which might ensue for him; but he considered that he who wished to work for the general good must not be deterred by apprehensions of danger. This interesting letter of consolation, which was written with much warmth, made so favorable an impression on the Jews of southern Arabia, that they, far from growing indifferent to their religion, were strengthened in it, and were moved to take an energetic share in all the events affecting the welfare of the whole body of Jews. In later times, when Maimuni attained greater importance, he found the means of putting a stop to the political oppression and bigoted persecution suffered by the Jews. For this the congregation of Yemen clove to him with enthusiastic love and veneration. They included his name in their daily prayer, a demonstration of honor which had been accorded only to the Exilarchs at their zenith.

Maimuni's greatness only gradually obtained acknowledgment. As early as the year 1175, he was looked upon as an authority in the determination of rabbinical laws; and religious-legal questions were addressed to him from all parts, a circumstance from which we may infer the universal recognition of his authority. Maimuni appears to have been officially recognized in 1177 as rabbi of Cairo, on account of his profound knowledge of the Talmud, his character, and his fame. He, with nine colleagues, formed an ecclesiastical board. His office he regarded as a holy priesthood, and exercised it with characteristic conscientiousness and circumspection. Where he perceived any abuses, he placed himself boldly in the breach. Although Maimuni worked hard in eliminating from the rabbinical world all Karaite customs which had crept in, he, nevertheless, always showed great tolerance toward the followers of Anan. Being asked how Rabbanites should behave towards Karaites, he replied that as long as they kept within the bounds of decency, and did not scoff at the Talmud, they were to be treated respectfully, and to be approached with friendliness, humility, and in a pacific spirit. Rabbanites might visit them in their houses, bury their dead, comfort their mourners, and initiate their children into the covenant of Abraham. The Talmud enjoins that we must observe a friendly demeanor towards heathens and idolaters, how much more so towards those who spring from the seed of Jacob, and acknowledge only one God. By virtue of his office, Maimuni tried hard to secure decorum in the synagogue, and also to remove many long-continued abuses. He noticed, for instance, that when the congregation had finished saying the silent prayer, thinking that they had performed their duty, they did not listen to its audible repetition by the reader, but chatted with one another, and generally behaved in an unbecoming manner. The Mahometans mocked at them, and with justice too, for they were accustomed to conduct their own divine service with concentrated devotion. Maimuni, who always felt deeply mortified when Judaism was exposed to ridicule, was anxious to put a stop to such offensive behavior in the synagogues, and with this motive abrogated the silent prayer altogether, without considering that it is expressly prescribed by the Talmud. Sincere prayer was to him of higher importance than mere mechanical fulfilment of precept. This practice, instituted by Maimuni, according to which the reader alone said the chief prayer, was followed, not only in the whole of Egypt, but even in several congregations of Palestine, in Damascus, and Haleb, and was continued among the native congregations for three centuries.