At the beginning of the Christian faith, there was no single set of texts or beliefs that united Christians. The four major gospels do not agree on everything, because they were written by different people from memory (decades after the apostles themselves were alive). It was St. Paul, a Jewish leader formerly named Saul who underwent a profound conversion experience and became the foremost Christian evangelist, who popularized the notion that Jesus's death on the cross was part of a divine plan that canceled out human sin. For hundreds of years, the Christians debated and argued about what Christ's message had “really” been, because many of Jesus's teachings were, and are, open to interpretation. Early Christians were divided on very significant issues, including:
What God did Jesus represent? One cult believed that the God of Christ was not the Jewish God, who had been vengeful and warlike; according to this sect, Christ's God was a more powerful and loving deity come to save the world from Yahweh.
Was Jesus the messiah? In Jewish doctrine, the messiah was to be a figure who liberated the Jews from oppression and made good on the Covenant between the Jews and God, delivering the Promised Land for all eternity. Many Jews had hoped that Jesus would be a revolutionary against Roman rule and, since Judea remained in Roman hands after his death, they did not believe that Jesus had been the messiah. Early Christians came to insist, following Paul, that Jesus had indeed been the messiah, but that the "liberation" he offered was spiritual in nature, rather than having to do with prosaic politics. In other words, the potential to save one's soul from damnation superseded the old Covenant.
Was Jesus human, or was he instead somehow God Himself? He had lived like a normal man, but according to the gospels he had also performed miracles, and he claimed to be the son of God. Likewise, while Jesus lived an exemplary life, he also displayed traits like anger and doubt (the latter most famously on the cross when he asked God why He had “forsaken” Jesus), traits that did not seem those of a “perfect” being. This debate would go on for centuries, with equally pious groups of Christians coming to completely different conclusions about Christ's divine and human natures.
Could everyone be a Christian, or was membership limited to the Jews? If Jesus was indeed the specifically Jewish messiah, after all, it did not make sense for a Roman or a Persian or a Celt to be able to convert. In the end, thanks largely to the influence of St. Paul again, most Christians came to believe that the salvation offered by Christ was potentially universal, and that not just Jews could become Christians as a result.
Under the influence of the mystery religions noted above, many early Christians were Gnostics, meaning "those who know" in Greek. The Gnostics believed that Jesus had been a secret-teller, almost a magician, who provided clues in his life and teachings about how to achieve union with God. This had more to do with magic than with a recognizable set of religious rituals or customs - for example, many Gnostics believed that it was possible to deduce a series of incantations from Christ's teachings that included hundreds of secret "names of God." If a Gnostic was to properly chant all of the names of God, he would not only achieve salvation but might enjoy power on earth, as well. The Gnostics had no interest in converting people to their version of Christianity; it was a secret they wanted to keep for themselves.
Still, despite the bewildering diversity of beliefs among early Christians, there were common themes, most importantly the emphasis Jesus Himself had placed on the spiritual needs of the common people, even social outcasts. The most radical aspect of Christianity was its universalism. From Judaism, it inherited the idea that all human beings are spiritually equal. Once the debate about whether non-Jews could become Christians was resolved, it was also potentially open to anyone who heard Christianity's teachings and doctrine. Early Christians recognized no social distinctions, which was fundamentally at odds with the entire Roman system, reliant as it was on formal legal separations between social classes and a stark system of social hierarchy. Likewise, one unequivocal requirement placed on Christians was to love their neighbors, meaning in practice showing kindness and compassion to others regardless of their social rank. Few concepts could have been more alien to Roman sensibilities.
Christianity thus at least potentially threatened the hierarchical nature of Roman society itself. Likewise, it inherited from Judaism a strict monotheism that refused to accept the worship of the Roman emperors. What made it even more threatening than Judaism, however, was that Christianity actively sought out new converts (i.e. Christianity was inherently evangelical, in stark contrast to Judaism which did not seek new members). Roman authorities were thus already very much inclined to be suspicious of the Christians as potential rabble-rousers. In 68 CE, Nero blamed the Christians for the huge fire that consumed much of the city of Rome, and hundreds of Christian were rounded up and slaughtered in the arena. The persecution of Christians became a potent symbol for Christianity as a whole; over a thousand years later when Christianity was firmly entrenched as the religion of Europe, the trope of martyrdom was still used to explain righteous suffering.