At about this time—in the second half of the eighth century—the Jews of Europe also emerged a little from the darkness which had covered them for centuries. Favored by the rulers, or at least neither ill-treated nor persecuted by them, they raised themselves to a certain degree of culture. Charlemagne, the founder of the empire of the Franks, to whom Europe owes its regeneration and partial emancipation from barbarism, also contributed to the spiritual and social advancement of the Jews in France and Germany. By the creation of the German-Frankish empire—which extended from the ocean to the further side of the Elbe, and from the Mediterranean to the North Sea—Charlemagne transferred the focus of history to Western Europe, whereas hitherto it had been at Constantinople, on the borderland between Eastern Europe and Asia. Although Charlemagne was a protector of the Church, and helped to found the supremacy of the papacy, and Hadrian, the contemporary Pope, was anything but friendly to the Jews, and repeatedly exhorted the Spanish bishops to prevent the Christians from associating with Jews and heathens (Arabs), Charlemagne was too far-seeing to share the prejudices of the clergy with respect to the Jews. In opposition to all the precepts of the Church and decisions of the councils, the first Frankish emperor favored the Jews of his empire, and turned to account the knowledge of a learned man of this race, who journeyed to Syria for him, and brought back to France the products of the East. While other monarchs punished the Jews for purchasing Church vessels or taking them as pledges from the clergy or the servants of the Church, Charlemagne adopted the opposite course; he inflicted heavy punishment on the sacrilegious ecclesiastics, and absolved the Jews from all penalties.

The Jews were at this period the principal representatives of the commerce of the world. While the nobles devoted themselves to the business of war, the commoners to trades, and the peasants and serfs to agriculture, the Jews, who were not liable to be called upon to perform military service, and possessed no feudal lands, turned their attention to the exportation and importation of goods and slaves, so that the favor extended to them by Charlemagne was, to a certain extent, a privilege accorded to a commercial company. They experienced only the restraint put upon all merchants in the corn and wine trade; the Emperor considered it dishonest to make a profit on the necessaries of life. This somewhat materialistic value set upon the Jews marks, however, great progress from the narrow-mindedness of the Merovingian monarchs, the Gunthrams and the Dagoberts, who saw nothing in the Jews but murderers of God. But Charlemagne also manifested deep interest in the spiritual advancement of the Jewish inhabitants of his empire. In the same way as he had cared for the education of the Germans and the French by inviting learned men from Italy, so also he earnestly desired to place a higher culture within the reach of the German and the French Jews. With this intention he removed a learned family, consisting of Kalonymos, his son Moses, and his nephew, from Lucca to Mayence (787), hoping besides to make the Jews independent of the academies of the Levant.

Charlemagne's embassy to the powerful Caliph Haroun Alrashid, to which was attached a Jew named Isaac, is familiar to every student of history (797). Although at first probably Isaac accompanied the two nobles, Landfried and Sigismund, only in the character of interpreter, he was nevertheless admitted into Charlemagne's diplomatic secrets. Thus, when the two principal ambassadors died on the journey, the Caliph's reply and the valuable presents which he had forwarded, fell into Isaac's sole charge, and he was received in solemn audience by the Emperor at Aix. The Emperor is also said to have requested the Caliph, through his embassy, to send him from Babylonia a learned Jew for his country, and Haroun is reported to have sent him a man answering his requirements. This man was a certain Machir, whom Charlemagne placed at the head of the Jewish congregation of Narbonne. Machir, who, like Kalonymos of Lucca, became the ancestor of a learned posterity, founded a Talmudical school at Narbonne.

Owing to their favorable position in the Frankish-German Empire, in which they held land, the Jews were permitted to undertake voyages and carry on business, and were harassed neither by the people nor by the really religious German ecclesiastics; they were also enabled to abandon themselves to their inclination for travel, and thus spread through many of the provinces of Germany. In the ninth century, numbers of them dwelt in the towns of Magdeburg, Merseburg, and Ratisbon. From these points, they penetrated further and further into the countries inhabited by the Slavonians on the further side of the Oder as far as Bohemia and Poland. Meanwhile, in spite of the favor which Charlemagne extended to them, he, like the best men of the Middle Ages, found it difficult to treat them on an entirely equal footing with the Christians. The chasm, which the Fathers of the Church had placed between Christianity and Judaism, and which had been widened by individual ecclesiastics and the synods, was far too deep to be overleapt by an emperor who was devotedly attached to the Church. Charlemagne himself maintained, on one point, a difference between Jew and Christian, and perpetuated it in the peculiar form of the oath which was imposed on the Jews who were witnesses against, or accusers of, a Christian. They were required, in taking an oath against a Christian, to surround themselves with thorns, to take the Torah in their right hand, and to call down upon themselves Naaman's leprosy and the punishment of Korah's faction in witness of the truth of their statement. If there was not a Hebrew copy of the Torah at hand, a Latin Bible was held to be sufficient. It is impossible not to admit, however, that to allow the Jews to testify against a Christian was in itself a deviation from the ordinances of the Church.

In the East, at the beginning of the ninth century, the Jews were also reminded, in a disagreeable manner, that they had to expect scorn and oppression even from the best rulers. The reigns of the Abassid Caliphs, Haroun Alrashid and his sons, are regarded as the most flourishing period of the Caliphate of the East, but it is at this very time that Jewish complaints of oppression rise loudest. It is possible that in re-enacting Omar's law against the Christians (807), Haroun also made it applicable to the Jews; for they were compelled to wear a distinctive badge of yellow on their dress, in the same way as the Christians were obliged to wear blue, and they had to use a rope instead of a girdle. When, after his death (809), his two sons, Mahomet Alemin and Abdallah Almamun, for whom their father had divided the Caliphate into two parts, engaged in a destructive civil war, throughout the whole extent of the great empire, the Jews, especially those in Palestine, experienced severe persecution. The Christians, however, were their companions in misfortune. During the four years (809–813) of this fratricidal struggle, robbery and massacre seem to have been the order of the day. The sufferings were so terrible, it seems, that a preacher of those times declared them to be a sign of the speedy coming of the Messiah. "Israel can only be redeemed by means of penitence, and true penitence can only be evoked by suffering, affliction, wandering, and want," declared this orator by way of consolation of his afflicted congregation. In the civil war raging between the two Caliphs, he fancied he saw the approaching destruction of the Ishmaelite rule and the approach of the Messianic empire. "Two brothers will finally rule over the Ishmaelites (Mahometans); there will then arise a descendant of David, and in the days of this king the Lord of Heaven will found a kingdom which shall never perish." "God will exterminate the sons of Esau (Byzantium), Israel's enemies, and also the sons of Ishmael, its adversaries." But these, like many others, were delusive hopes. The civil war, indeed, shook the Caliphate to its foundations, but did not destroy it. Alemin was killed, and Almamun became the sole ruler of this extensive empire.

It was during Almamun's reign (813–833) that the Caliphate of the East flourished most luxuriantly. As he was imbued with tolerance, it was possible for the sciences and a certain form of philosophy to develop. Bagdad, Kairuan in northern Africa, and Merv in Khorasan, became the centers of science, such as Europe did not possess until many centuries later. The genius of the Greeks celebrated its resurrection in Arabic garb. Statesmen competed with men of leisure for the palm of erudition. The Jews did not remain unaffected by this enthusiasm for science. Investigation and subtle inquiry are indeed part of their innermost nature. They took earnest interest in these intellectual activities, and many of their achievements gained the approbation of the Arabs. The history of Arab civilization has several Jewish names recorded in its annals. Sahal, surnamed Rabban (the Rabbanite, the authority on the Talmud), of Taberistan on the Caspian Sea (about the year 800), was celebrated as a physician and a mathematician. He translated into Arabic the Almagest of the Greek astronomer Ptolemy, the text-book of astronomy during the Middle Ages, and was the first to note the refraction of light. His son Abu-Sahal Ali (835–853) is placed among those that advanced the study of medicine, and was the teacher of two Arabic medical authorities, Razi and Anzarbi.

With even more ardor than that with which they had applied themselves to medicine, mathematics and astronomy, the Mussulmans prosecuted the study of the science of religion, a sort of philosophy of religion (Kalâm). It was invested with as much importance as the affairs of state, and exercised a certain influence on politics. The expounders of the Koran, in trying to explain away the grossly sensual references to God, and to reconcile the contradictions contained in that work, developed ideas which projected far beyond the restricted horizon of Islam. Many commentators, by reason of their rationalistic explanations, came into conflict with the champions of the text, and were branded by them as heretics. The Mutazilists (heretics) laid great stress upon the unity of God, and desired that no definite attributes should be ascribed to him; for thereby the essence of God appeared to them to be divided into parts, and several beings to be included in the idea of God, whose unity was thus negatived. They further asserted the freedom of the human will, because the unconditional predetermination by God, which the Oriental mind believes, and the Koran confirms, was incompatible with divine justice, which rewards the good and punishes the bad. They believed, however, that they still stood on the same ground as the Koran, although, of course, going far beyond it, and in order to bring their doctrine into harmony with the blunt sayings of their religious book, they employed the same method as the Alexandrian-Jewish philosophers of religion had used to reconcile the Bible with Greek philosophy; they adopted an allegorical interpretation of the text. This interpretation was employed for the purpose of bridging over the gulf existing between the rationalistic idea of God and the irrational idea as taught by the Koran. The rationalistic Mutazilist theology of the Mahometans, although denounced at first as heretical, steadily gained ascendancy; the schools of Bagdad and Bassora rang with its doctrines. The Caliph Almamun exalted it into the theology of the court, and condemned the old simple views of religion.

The adherents of orthodoxy were horrified by this license of interpretation, for the text of the Koran, in an underhand way, was forced into conveying an opposite meaning, and simple faith lost all support. They, therefore, adhered strictly to the letter and to the natural meaning of the text. Some of them went still further. They took, in their literal meaning, all the expressions concerning God, however gross they might be, which occurred in the Koran, or were used by tradition, and constructed a most vile theology. Mahomet expressed a revelation thus: "My Lord came to meet me, gave me his hand in greeting, looked into my face, laid his hand between my shoulders, so that I felt his cold finger-tips," and the orthodox school accepted all this in revolting literalness. This school (Anthropomorphists) did not hesitate to declare that God was a body possessed of members and a definite form; that he was seven spans high, measured by his own span; that he was in a particular spot—upon his throne; that it was permissible to affirm of him that he moves, mounts his throne and descends from it, stops and rests. These and still more blasphemous descriptions of the Supreme Being, in the same grossly materialistic strain, were given by the orthodox Mahometan teachers of religion, in order to show their adherence to the letter of the Koran in contradistinction to the Rationalists.

The Jews of the East lived in so close a connection with the Mussulmans that they could not fail to be affected by these tendencies. The same phenomena were repeated, therefore, in Jewish circles, and the variance between Karaites and Rabbanites assisted in transferring the Islamic controversies to Judaism. The official supporters of Judaism, however, the colleges of Sora and Pumbeditha, held aloof from them. Entirely absorbed in the Talmud, and its exposition, they either took no notice at first of the violent agitation of mind prevailing, or else refused to yield to it. But outside of the colleges men were actively interested in these new methods, and Judaism was pushed through another process of purification.

The faint ray of philosophy which fell into this world of simple blind faith, ignorant of its own beliefs, produced a dazzling illumination. The Karaites for the most part were of Mutazilist (rationalistic) tendency, while the Rabbanites, on the contrary, having to defend the strange Agadic statements concerning God, were antagonistic to science. But as the religious edifice of Karaism was not finished, there arose new sects within its pale, with peculiar theories and varying religious practices.