The Islamic Reformation was the series of events in European and Middle Eastern history which resulted in the conversion of the Roman state church, and many other national churches, from Christianity to Islam. It does not refer to any specific period of history, but was instead a protracted affair which affected different regions at different times.
For the most part, the process was relatively peaceful, with the existing institutions and organizational structures simply being adapted for a different faith, though there were a number of religious wars and persecutions on both sides.
After those of Arabia itself, some of the first Christians to accept Islam were the Monophysites of Egypt and Syria. They were greatly aided in this by the early conversion of the Emperor Heraclius and his efforts at the Third Council of Constantinople to encourage acceptance.
However, despite official recognition, the process of conversion was slow and uncertain. The majority of the population, particularly in Italy, Africa, Greece and Anatolia, adhered to the Chalcedonian creed, and their bishops were strongly opposed to any change. Immediately after the Council these regions rose up in revolt against Heraclius, and only by making compromises and agreeing to scale back his plans was he able to subdue the rebellion.
Progress was slow after this, but by the reign of Constantine IV it had been common for the educated to, at least nominally, convert in the hopes of preferment for civil or military advancement. Constantine's son Justinian II was the last Christian emperor and attempted to undo many of the changes of the last decades, but he was overthrown before this could be realised. It was Tiberius III who made it clear that Islam was here to stay, and so ended the last serious resistance to the new order in the east.
After this the Islamification question was effectively resolved, but there still remained a debate to be had on how much of the old Christian culture should be preserved. Among other things, there remained thousands of religious icons in churches and mosques, despite the explicit Quranic prohibition of such images. Leo III attempted to destroy such images, beginning what became known as the iconoclasm, but the backlash was such that his successors eventually had to back down and allow some icons to be tolerated.
By the year 900 the position of Islam in the Roman Empire was secure, and the country had become a source for missionaries to other parts of Europe and beyond.
For many years the church and aristocracy of the west were able to resist change, thanks to the efforts of the Pope in Rome. The rulers of Italy and Africa ruthlessly suppressed any trace of Islamic thought to prevent it from becoming established. The Bulgar conquest of the Balkans cut the land route between the eastern and western empires, thus preventing any Roman interference.
This system persisted until 867, when the then King of Africa was elected Emperor and converted to Islam. Within a few generations the people of Africa were well on the way towards converting, and merchants and missionaries were able to journey from the east to Spain and Gaul via Africa without having to pass through papal territory. At this time the papacy was under Frankish domination and thereby discredited in the eyes of many non-Franks, so it did not take long for the new religion to find willing ears. In 888 Alfonso II of Spain publically converted and proclaimed the separation of the Church of Spain from that of Rome, though the Spanish church as a whole did not convert until much later.
Italy was the last Christian bastion in the western Mediterranean, and for most of this time was a theocratic state under the rule of the Pope. This did not change until the early 12th century when Emperor Alexios I, in retaliation for the depredations of the Great Crusade, invaded Italy and annexed it to the Roman Empire. Missionaries were soon active in Italy, and most of the population had converted by the year 1500.
Central and Northern Europe
The Islamification of the remainder of Europe, including the Albic Isles, the Germanies, and Gaul, progressed from several different directions.
From about 900, Spanish missionaries were active in Aquitaine, intensifying after the region was reconquered by Spain in 958. They also journeyed along the coast to Arvor and Albion. In Arvor Islam was soon welcomed by the majority of the population, but for years it was rejected by the people of the Albic Isles, who were fond of their monasteries and opposed the Islamic ban on monasticism. Islam was eventually established in the isles after the 1078 Arvorian conquest of Prydain, and in Lyonesse when that kingdom became a part of the Jersiais empire in 1328. Both country's national churches had already schismed from Rome, and so the change in doctrine was able to be introduced relatively peacefully.
In the Germanies, Islam took longer to take hold thanks to the religious policies of the Holy Roman Emperors and their puppet Popes of Mainz, but by 1400 Germany was completely surrounded by Muslim-majority countries and unable to resist their influence. The Bohemian priest and preacher Jan Hus was instrumental in having the religion accepted within the empire, and his efforts were sufficient to convert the rulers and populace of Bohemia, Swabia and several other regions. The Treaty of Prague of 1424 brought a temporary solution to the crisis, but tensions continued to grow resulting in the outbrek of the Forty Years' War the following century. After this war finally ended, freedom of religion was instituted all across the Holy Roman Empire, with the result that Muslims now make up most of the population of the southern and central regions.
Unlike the remainder of Europe, most of the Slavic, Finnic and Baltic peoples of the east converted to Islam directly from their ancestral pagan religions without first passing through a Christian phase. Therefore, it is not correct to speak of there being a "reformation" in the same sense as elsewhere.
Islam was first established in this part of the world when Vladimir I of the Rus' agreed to convert as part of a treaty of alliance with the Roman Empire. According to legend, there were initially some obstacles when Vladimir was reluctant to accept certain restrictions, particularly on alcohol, but these were overcome when the Caliph agreed that such restrictions were not neccessary for belief. The religion spread rapidly through the Rus' lands, and from there into Finland, Lithuania and Poland after the destruction caused by the Mongol invasions.
Islam was present in Abyssinia almost from the very beginning. According to legend, when some of the very first Muslims in Mecca faced persecution from the ruling Quraysh, they were advised by Muhammad to seek shelter overseas and were welcomed by Ella Seham, King of Axum. Thereafter, Axum enjoyed friendly relations with the early Muslim community and allowed missionaries to travel freely.
Ella Seham's second son, Wasan Sagad, was the first Abyssinian king to publically convert, doing so in the aftermath of the Persian war where he fought alongside the Caliphate and the Roman Empire. The new religion spread rapidly amongst the populace, both in Axum itself and in its many vassal kingdoms, and from there ships carrying missionaries were able to reach as far as Zanzibar in the south and the Malayan islands in the east.