Unlike today, Arabia at the time of Mohammed had a sizable and well-established Jewish population and in fact some cities (such as Yathrib – modern-day Madina – had several Jewish ruling tribes). This was the result of several waves of immigration over the centuries; every time there was turbulence or persecution in Judea and Samaria, more Jews would escape to the Arabian Peninsula to the south. And so by the 7th century AD, Jewish communities had settled throughout the region. They mixed and traded with Arab tribes, but in keeping with both their own customs and those of the local region they rarely intermarried and while settled and well-respected, they did not assimilate into the local Arab culture.
It seems to have been generally believed that the Jewish arrivals in Arabia shared a common ancestry with the existing residents through Abraham’s children Isaac and Ishmael. While there is no actual evidence that the Arabs were descendants of Ishmael, the account in Genesis of Ishmael’s journey south to the Desert of Paran – close to the northern Arabian Peninsula – led to the assumption that the Arabs in the peninsula at that time were his descendants. While the newly-arrived Jews were not particularly interested in forging close ties with the Arabs based on their supposed kinship, it was to their benefit to promote this idea as it would have afforded them some degree of protection according to the local honour code. As a result, by the time of the birth of Mohammed, the idea of cousinship between the Arabs and the Jews was taken for granted by almost everyone.
One result of the many largely independent Jewish communities which had taken root over the years was the development of some very disparate sets of beliefs, many of which had moved significantly away from the orthodoxy of the Old Testament. Another was the fact that the Arabs of the time would have often come into contact with these Jewish communities, and would have had at least a passing familiarity with their various beliefs. The Jews who lived in Arabia were waiting for the coming of the Messiah, the king promised in the Old Testament, the one who was going to free them from oppression and take them back to the Promised Land. Their stories about the coming of the Messiah thus spread through the Arab community, and the locals too began to expect a coming Messiah or prophet which may have paved the way for the acceptance of Mohammed and his message of monotheism.