The cosmopolitan city of Baghdad, at this…

The cosmopolitan city of Baghdad, at this time the world’s largest, has blossomed during al-Mahdi's reign.

The city attracts immigrants from all of Arabia, Iraq, Syria, Persia, and lands as far away as India and Spain.

Baghdad is home to Christians, Jews, Hindus, and Zoroastrians, in addition to the growing Muslim population.

Al-Mahdi has continued to expand the Abbasid administration, creating new diwans, or departments, for the army, the chancery, and taxation.

Qadis or judges have been appointed, and laws against non-Arabs have been dropped.

The Barmakid family staffed these new departments.

The Barmakids, of Persian extraction, had originally been Buddhists, but shortly before the arrival of the Arabs, they had converted to Zoroastrianism.

Their short-lived Islamic legacy will count against them during the reign of Harun al-Rashid.

The introduction of paper from China after 751, which had not yet been used in the West—the Arabs and Persians used papyrus, and the Europeans used vellum—had a profound effect.

The paper industry boomed in Baghdad where an entire street in the city center becomes devoted to sales of paper and books.

The cheapness and durability of paper are vital to the efficient growth of the expanding Abbasid bureaucracy.

Al-Mahdi has two important religious policies: the persecution of the zanadiqa, or dualists, and the declaration of orthodoxy.

Al-Mahdi had singled out the persecution of the zanadiqa in order to improve his standing among the purist Shi'i, who want a harder line on heresies, and find the spread of syncretic Muslim-polytheist sects to be particularly virulent.

Al-Mahdi has declared that the caliph has the ability—and indeed, the responsibility—to define the orthodox theology of Muslims, in order to protect the umma against heresy.

Although al-Mahdi does not make great use of this broad, new power, it will become important during the 'mihna' crisis of al-Ma'mun's reign.