Legitimacy, and especially the science of creed, was dear to him. Therefore, he hastened to author books for his followers on Tawhid (monotheism) specifically, in which he sought an easy and accessible style for scholars. He authored some of them in the Berber language, which the people spoke, as some of them did not master the Arabic language. Ibn Khaldun said: "He descended to his people in the year 515 AH and built a lodge for worship. The youth and tribes gathered to him, and he taught them guidance in Tawhid in the Berber language." He presented a volume containing knowledge of God Almighty, knowledge of the reality of divine decree and destiny, faith, Islam, attributes, what is incumbent upon God Almighty, what is impossible for Him, and what is permissible for Him. And faith in what He has informed of His unseen, and from the principles of religion, and knowledge of the Mahdi, and that he is the Imam, and the obligation of the Imamate, and what is due to him of reverence and veneration. And that migration to him is obligatory, and no Muslim, family, child, or wealth can stand between him and it. And whoever hears of his command, migration to him is obligatory. And he declared those who do not pray for him and obey him to be infidels. He mentioned to them the etiquette among them, the sign of a believer, and what the believer must do of enjoining good and forbidding evil. He clarified them in the clearest way. It appears from these topics that the book contained the most important foundations upon which the Mahdi intended to establish the society, whether in the social sphere or otherwise. It was named the Book of Tawhid, referring to the concept of Tawhid in its various dimensions of creed. The Mahdi mastered the Berber language, in addition to the Arabic language, and used them in his sermons and writings according to the circumstances. (See Ibn Khaldun, Kitab al-'Ibar, vol. 6, p. 125. Also see Al-Zarkani, Manahil al-'Irfaan, vol. 2, p. 125. Ibn al-Qattan, Nazm al-Juman, p. 125. Al-Maqrizi, Kitab al-Khutal, p. 125.)