Medieval and modern Christian theology is a product of both the jewish and the Greek traditions. The synthesis began not among Christians but among Jews, especially those who had migrated in large numbers to the Greco-Egyptian metropolis of Alexandria. Here Jewish scholars - in particular a religious philosopher of the early first century A.D named Philo Judaeus - worked toward the reconciliation of Jewish Biblical revelation and Greek philosophy that was to influence both Jewish and Christian thought across the centuries.

Following the lead of Philo Judaeus, Christian theologians strove to demonstrate that their religion was more than merely an appealing myth - that it could hold its own in the highest intellectual circles. Plato and the Bible agreed, so they argued, on the existence of a single God and the importance of living an ethical life. But as Christians explored their faith more analytically, they began to differ among themselves on such difficult issues as the nature of Christ (how could he be both God and man?) and the Trinity (how can three be one?). Some opinions were so inconsistent with the prevailing view that they were condemned as "heresies". As questions were raised and orthodox solutions agreed on, Christian doctrine became increasingly specific and elaborate.

The early heresies sought to simplify the nature of Christ and the Trinity. One group, the Gnostics, insisted that Christ was not truly human but only a divine phantom - that God could not have degraded himself by assuming a flesh-and-blood body. Others maintained that Christ was not fully divine, not an equal member of the Trinity. This last position was taken up in the fourth century by a group of Christians known as Arians (after their leader, Arius), who spread their view throughout the Empire and beyond.

The orthodox position lay midway between Gnosticism and Arianism: Christ was fully human and fully divine. He was a coequal member of the Holy Trinity who had always existed and always would, but who had assumed human form and flesh at a particular moment in time and had walked the earth, taught, suffered and died as the man Jesus.

It was in this increasingly supernatural atmosphere that the Christians converted the Roman Empire. Some of their beliefs and practices resembled those of older and competing religions: baptism, eternal salvation, the death and resurrection of a savior-god, the sacramental meal, human brotherhood under a divine father; none of these was new. Yet Christianity was more than a recombination of old beliefs, more than simply another of the mystery religions. It differed from them above all in two fundamental ways: (1) its founder and savior was an actual historical personage: compared with Jesus such mythical idealizations as Isis and Mithras would have seemed faint and unreal; (2) its god was not merely the best of many gods but the One God, the God of the Jews, unique in all antiquity in his claims to exclusiveness and omnipotence, and now detached by Christianity from his association with the Jewish people to become the God of all peoples.

According to Gospels, Jesus' greatest miracles was his resurrection - his return to life on the third day after his death on the cross. he is said to have remained on earth for a short period thereafter, giving solace and instruction to his disciples, and then to have ascended into heaven with the promise that he would return in glory to judge all souls and bring the world to an end. The early generations of Christians expected this second coming to occur quickly, which may be one of the reasons why formal organization was not stressed in the primitive Church.

From the beginning, Christians not only accepted Jesus' ethical teachings but also worshiped him as the Christ, the incarnation of God. In the Gospels, Jesus distinguishes repeatedly between himself - "the Son of Man" - and God - "the Father" - but he also makes the statement, "I and the father are one". And in one account he instructs his followers to baptize all persons "in the name of the Father and the Son and the Holy Spirit". Hence, Christianity became committed to the difficult and sophisticated nation of a single divinity with three aspects. Christ was the "Son" or "Second Person" in a Holy Trinity that was nevertheless one God. The doctrine of the Trinity produced a great deal of theological controversy over the centuries. But it also gave Christians the unique advantage of a single, infinite, philosophically respectable God who could be worshiped and adored in the person of the charismatic, lovable, tragic Jesus.