Church historian Kenneth Scott Latourette considers the question why Christianity spread as no other religion had spread before it.
There were many other religions and philosophies. But none attracted the following Christianity did or had the impact Jesus' church did. Why?
Inevitably the question arises: Why, from being the faith of a small, persecuted minority in competition with other religions which appeared to have better prospects of success, did Christianity eventually enroll the large majority of the population of the Roman Empire? To that outcome several factors contributed. In the disintegration of the existing order which by the end of the second century was becoming obvious many individuals were seeking spiritual and material security and believed that they could find it in the Christian faith. By the end of the third century, while enlisting only a minority, the Church was Empire-wide, was more comprehensive than any institution except the state, and gave to its members a sense of brotherhood and solidarity. Christianity assured its adherents what many in the ancient world were craving -- high ethical standards, a spiritual dynamic in which was power to approximate to those standards, and immortality. The Church was inclusive: its brotherhood included both sexes, rich and poor, intelligentsia and men and women of no intellectual attainments. Many intellectuals, including Augustine, found in the faith not only moral power but also, in the incarnation, the Word become flesh, what was absent in the highest philosophies of the time. The constancy of the martyrs awakened the admiration of thousands. So did the fact that Christianity was uncompromising in its demands. One modern scholar, T. R. Glover of Cambridge University declared that the Christians out-thought, out-lived, and out-died the adherents of the non-Christian religions.
The primary source of the appeal of Christianity was Jesus -- His incarnation, His life, His crucifixion, and His resurrection. Here was the sense of security and of meaning in a perplexing universe and in a society whose foundations were crumbling. Here were the command for and, although imperfect, the realization of a comprehensive fellowship. Here were high and exacting ethical commands and the proved power to approximate to them. Here was victory through apparent defeat. Here was the certainty of immortality in ever-growing and never-ending fellowship with the eternal God Who so loved that He had given Himself in His Son.
Because Christianity made no distinctions in the value of human beings. Many philosophies of the time appealed to the educated. Other religions viewed man as instrument of the gods. Christianity was a unique religion. God loved mankind and came in the flesh to sacrifice Himself for mankind; He didn't expect sacrifices to appease Him. He valued all people, not for their status or education or race or sex or accomplishment, but simply because they were His creation. And His church was to live these values as He transformed them. Christianity, unique among all religions and philosophies, was expansive in accepting people who shared their worship of Jesus as Savior.
Christianity was unique. And its impact on the world is hard to apprehend now because we take for granted the values grounded in it. Christians invented hospitals, orphanages, and social programs because they loved each other and others because God loved them and commanded them to. Christianity established equal human value despite differences based on simply being God's creature.
These kinds of values are commonplace now, but they were spawned by the Christian worldview and still find their home in Biblical values.