Islam, like any major religion, is a complex phenomenon. Diverse, at times even contradictory, it resists summary and categorical description. The religion was born among the inhabitants of the Arabian Peninsula in the early seventh century CE. Yet Islam as we know it is the product of many peoples and cultures: the Arabs, but also converts from among the Jewish, Christian and Zoroastrian communities of the Near East. By 1800 Islam had expanded into south-eastern Europe, central, south and South-East Asia, and sub-Saharan Africa. Indeed, Islam formed what many have called the first global civilisation. Its ecumenical reach finds inspiration in a famous verse from the Qurʾān: ‘Thus we have appointed you a middle nation, that you may be witnesses to mankind’ (Q 2:143). As a historical matter, however, Islam is best understood as an expression of the larger tradition of Near Eastern monotheism. It is distinct from its older cousins, Judaism and Christianity, but its origins and early development owe much to them.

The historical relationship of Islam and Christianity is especially fraught. From the beginning, Muslims were aware of Christianity. Several of their core beliefs were constructed as a response to or reaction against Christian doctrine. The Muslim state took shape in the context of an existential struggle against the Byzantine empire, which understood itself to be the defender of the Church. Over the ensuing centuries the competition between Christianity and Islam took ever sharper forms. In the Middle East, at least, most of those who eventually converted to Islam did so from Christianity, a fact that underlies the intense competition between the two religions. The competition only worsened, and grew more violent, with the rise of the Crusading movement in the late fifth/eleventh and sixth/twelfth centuries.