The Muslims
But in nomadic Arabia, Muhammad was thought to have had revelations from God, which altered what was known from the Hebrew and Christian Scriptures, and introduced a new religion which in many ways was antithetical to Christianity. There was but one God, and in no sense a Trinity; Jesus was a prophet, the second of three great prophets: Moses, Jesus, and Muhammad. He was not really killed on a cross, and certainly his revelation was not final.
The new religion was much more belligerent than Christianity, which had emphasized letting others trample upon one's rights. This religion was more of a "natural" religion, a seeing into the truth that was there in nature; but it included an afterlife of reward for those who followed its dictates and a punishment for those who did not. It not only permitted fighting, but endorsed wars for the sake of religion; and it allowed vengeance up to four times back and forth for wrongs committed. Further, it permitted polygamy, and put women into a position where they were not to be seen in public.
While it stressed brotherhood and tolerance, it was still the case that it regarded those who did not have the faith not merely as unenlightened and pitiable, but as also the enemy of God, and people who were, by force if necessary, to be brought into conformity with the truth. The Christians particularly were enemies, because with their Trinitarian view of God, they committed the blasphemy of actually saying that Jesus was Allah; and this insult to God could not be permitted to happen. Unlike the Indian religion and philosophy, with its notion of a divine core in everything, Islam in principle was intolerant of any belief that denied what it held Allah to be, however tolerant the Muslims themselves might be.
Given that war was the order of the day, this religion struck a responsive chord in many people, especially the despised nomads at the eastern fringes of the early Roman empire. It spread by conquest over that area of the world, but the conquered were conquered spiritually as well as materially; and even the Eastern Roman Empire succumbed. Then Africa and Spain and even Sicily fell under the domination of the Muslims, though in Europe Islam did not really replace Christianity, but made at least a temporary accommodation with it.
The dominant position of Islam in its part of the world led to a good deal of leisure among the ruling Muslims; and scholars began studying the works that had survived from ancient times. It was the Arabs who gave the world the number system we now use, and who modified astronomy from Aristotle's rather crude system of spheres to the Ptolmaic view of the world; and the Muslim philosophers did significant work on Plato and especially Aristotle, whose writings (except for the logical works) had fallen into oblivion because they seemed so irreconcilable with Christianity.
They were also irreconcilable with Islam, however; and in fact, much of what was in the Koran was not something very compatible with what was being learned from the natural science researches of people like Aristotle. Thinkers like Averroes got away with continuing to study these pagan philosophers on the grounds that the Koran was Theological truth, which was believed, and Aristotle was philosophical truth, which was known; and both could be true at the same time, even if they contradicted each other, since they were truths of different orders.
Meanwhile in the West, the encroachments of Islam became a great worry; and its taking over the Holy Land an outrage that galvanized the fragmented people into unifying at least to the extent of sending an expedition to the Holy Land to free it; and so the Crusades occurred, with the military clash between a more or less united Christendom and Islam.
The Crusades brought people of many places together in common cause, and accelerated the process (which had already been going on) of unifying the little dukedoms into kingdoms. There was even, with Charlemagne, the attempt to revive the Roman Empire under a Christian umbrella, with the Emperor subject in some sense to the Pope.
So the Christian "we," faced with a threat from outside, began to expand.
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