Although the differences that divide these schools at first caused great excitement and gave rise to violent discussions, the strong catholic instinct of Islâm always knew how to prevent schism. Each new generation either found the golden mean between the extremes which had divided the preceding one, or it recognized the right of both opinions.

Though the dogmatic differences were not necessarily so dangerous to unity as were political ones, yet they were more apt to cause schism than discussions about the law. It was essential to put an end to dissension concerning the theological roots of the whole system of Islâm. Mohammed had never expressed any truth in dogmatic form; all systematic thinking was foreign to his nature. It was again the non-Arabic Moslims, especially those of Christian origin, who suggested such doctrinal questions. At first they met with a vehement opposition that condemned all dogmatic discussion as a novelty of the Devil. In the long run, however, the contest of the conservatives against specially objectionable features of the dogmatists' discussions forced them to borrow arms from the dogmatic arsenal. Hence a method with a peculiar terminology came in vogue, to which even the boldest imagination could not ascribe any connection with the Sunnah of Mohammed. Yet some traditions ventured to put prophetic warnings on Mohammed's lips against dogmatic innovations that were sure to arise, and to make him pronounce the names of a couple of future sects. But no one dared to make the Prophet preach an orthodox system of dogmatics resulting from the controversies of several centuries, all the terms of which were foreign to the Arabic speech of Mohammed's time.

Indeed, all the subjects which had given rise to dogmatic controversy in the Christian Church, except some too specifically Christian, were discussed by the mutakallims, the dogmatists of Islâm. Free will or predestination; God omnipotent, or first of all just and holy; God's word created by Him, or sharing His eternity; God one in this sense, that His being admitted of no plurality of qualities, or possessed of qualities, which in all eternity are inherent in His being; in the world to come only bliss and doom, or also an intermediate state for the neutral. We might continue the enumeration and always show to the Christian church-historian or theologian old acquaintances in Moslim garb. That is why Maracci and Reland could understand Jews and Christians yielding to the temptation of joining Islâm, and that also explains why Catholic and Protestant dogmatists could accuse each other of Crypto-mohammedanism.

Not until the beginning of the tenth century A.D. did the orthodox Mohammedan dogma begin to emerge from the clash of opinions into its definite shape. The Mu'tazilites had advocated man's free will; had given prominence to justice and holiness in their conception of God, had denied distinct qualities in God and the eternity of God's Word; had accepted a place for the neutral between Paradise and Hell; and for some time the favour of the powers in authority seemed to assure the victory of their system. Al-Ash'arî contradicted all these points, and his system has in the end been adopted by the great majority. The Mu'tazilite doctrines for a long time still enthralled many minds, but they ended by taking refuge in the political heresy of Shî'itism. In the most conservative circles, opponents to all speculation were never wanting; but they were obliged unconsciously to make large concessions to systematic thought; for in the Moslim world as elsewhere religious belief without dogma had become as impossible as breathing is without air.

Thus, in Islâm, a whole system, which could not even pretend to draw its authority from the Sunnah, had come to be accepted. It was not difficult to justify this deviation from the orthodox abhorrence against novelties. Islâm has always looked at the world in a pessimistic way, a view expressed in numberless prophetic sayings. The world is bad and will become worse and worse. Religion and morality will have to wage an ever more hopeless war against unbelief, against heresy and ungodly ways of living. While this is surely no reason for entering into any compromise with doctrines which depart but a hair's breadth from Qorân and Sunnah, it necessitates methods of defence against heresy as unknown in Mohammed's time as heresy itself. "Necessity knows no law" is a principle fully accepted in Islam; and heresy is an enemy of the faith that can only be defeated with dialectic weapons. So the religious truths preached by Mohammed have not been altered in any way; but under the stress of necessity they have been clad in modern armour, which has somewhat changed their aspect.

Moreover, Islâm has a theory, which alone is sufficient to justify the whole later development of doctrine as well as of law. This theory, whose importance for the system can hardly be overestimated, and which, nevertheless, has until very recent times constantly been overlooked by Western students of Islâm, finds its classical expression in the following words, put into the mouth of Mohammed: "My community will never agree in an error." In terms more familiar to us, this means that the Mohammedan Church taken as a whole is infallible; that all the decisions on matters practical or theoretical, on which it is agreed, are binding upon its members. Nowhere else is the catholic instinct of Islâm more clearly expressed.

A faithful Mohammedan student, after having struggled through a handbook of law, may be vexed by a doubt as to whether these endless casuistic precepts have been rightly deduced from the Qorân and the Sacred Tradition. His doubt, however, will at once be silenced, if he bears in mind that Allah speaks more plainly to him by this infallible Agreement (Ijmâ') of the Community than through Qorân and Tradition; nay, that the contents of both those sacred sources, without this perfect intermediary, would be to a great extent unintelligible to him. Even the differences between the schools of law may be based on this theory of the Ijmâ'; for, does not the infallible Agreement of the Community teach us that a certain diversity of opinion is a merciful gift of God? It was through the Agreement that dogmatic speculations as well as minute discussions about points of law became legitimate. The stamp of Ijmâ' was essential to every rule of faith and life, to all manners and customs.

All sorts of religious ideas and practices, which could not possibly be deduced from Mohammed's message, entered the Moslim world by the permission of Ijmâ'. Here we need think only of mysticism and of the cult of saints.

Some passages of the Qorân may perhaps be interpreted in such a way that we hear the subtler strings of religious emotion vibrating in them. The chief impression that Mohammed's Allah makes before the Hijrah is that of awful majesty, at which men tremble from afar; they fear His punishment, dare hardly be sure of His reward, and hope much from His mercy. This impression is a lasting one; but, after the Hijrah, Allah is also heard quietly reasoning with His obedient servants, giving them advice and commands, which they have to follow in order to frustrate all resistance to His authority and to deserve His satisfaction. He is always the Lord, the King of the world, who speaks to His humble servants. But the lamp which Allah had caused Mohammed to hold up to guide mankind with its light, was raised higher and higher after the Prophet's death, in order to shed its light over an ever increasing part of humanity. This was not possible, however, without its reservoir being replenished with all the different kinds of oil that had from time immemorial given light to those different nations. The oil of mysticism came from Christian circles, and its Neo-Platonic origin was quite unmistakable; Persia and India also contributed to it. There were those who, by asceticism, by different methods of mortifying the flesh, liberated the spirit that it might rise and become united with the origin of all being; to such an extent, that with some the profession of faith was reduced to the blasphemous exclamation: "I am Allah." Others tried to become free from the sphere of the material and the temporal by certain methods of thought, combined or not combined with asceticism. Here the necessity of guidance was felt, and congregations came into existence, whose purpose it was to permit large groups of people under the leadership of their sheikhs, to participate simultaneously in the mystic union. The influence which spread most widely was that of leaders like Ghazâlî, the Father of the later Mohammedan Church, who recommended moral purification of the soul as the only way by which men should come nearer to God. His mysticism wished to avoid the danger of pantheism, to which so many others were led by their contemplations, and which so often engendered disregard of the revealed law, or even of morality. Some wanted to pass over the gap between the Creator and the created along a bridge of contemplation; and so, driven by the fire of sublime passion, precipitate themselves towards the object of their love, in a kind of rapture, which poets compare with intoxication. The evil world said that the impossibility to accomplish this heavenly union often induced those people to imitate it for the time being with the earthly means of wine and the indulgence in sensual love.

Characteristic of all these sorts of mysticism is their esoteric pride. All those emotions are meant only for a small number of chosen ones. Even Ghazâlî's ethical mysticism is not for the multitude. The development of Islâm as a whole, from the Hijrah on, has always been greater in breadth than in depth; and, consequently, its pedagogics have remained defective. Even some of the noblest minds in Islâm restrict true religious life to an aristocracy, and accept the ignorance of the multitude as an irremediable evil.