The key figure in the early years was Saul of Tarsus, better known as St. Paul. St. Paul, a Jew and Roman citizen, lived after Jesus' death. At one time he actively participated in persecuting Christians, but, after a dramatic conversion experience, he became by far the most influential figure in establishing Christianity as a religion not just for Jews but for all. He did so by constantly traveling to energize and support the budding urban Christian communities dotting the Mediterranean world, but also by shaping Christian thought through his letters. That thought had as its essence two main ideas. The first was the idea that Jesus lives not in the sense of still walking about, preaching and healing, but in the sense that he lives within those who have taken him into themselves, as Paul felt that he had taken Jesus into himself. The message of Paul was, then, a message not about theology but about personal experience, including his own and those of others as well.
The other main idea was about love. While, as pointed out above, Jesus' own preaching and actions make clear that the mystery of God is bound up with the mystery of love, Paul led the community of Christians in a direction that, centuries later, culminated in the formulation of the Nicene and Apostles' creeds that, for many, define what Christians believe. We end this short summary, then, with reference to the doctrine of the Trinity, which states that the divine is best (although inadequately) defined as having three parts that are paradoxically one. Those parts are "The Father" (God), "Son" (Jesus), and "Holy Spirit." In the doctrine of the Trinity, the early Christian Church found a way to combat emerging heresies. However, the doctrine did much more. It provided a way to succinctly define the Christian experience.
To the outsider, the doctrine of the Trinity appears nonsensical, a product of illogical if not primitive magical thinking. However, to those who had experienced the transforming power of "letting go and letting God" and of living the life of faith, the doctrine of the Trinity made perfect sense. To the early Christians, their own experience was best defined as having a personal connection to Jesus, God, and a spirit felt in the fellowship of their Christian community.
The doctrine of the Trinity brings us back to perhaps the core message of the Christian faith tradition, or to what is known as its gospel or "Good News." That good news, according to Christians, is that God's true, loving nature was revealed in the life and death of Jesus; that Jesus is God, which makes his example binding on us all; and that Jesus is fully human, which makes his example relevant to all. In addition, the good news is that God and Jesus live as a Holy Spirit, which makes the Christian community into a community of diverse parts of the same "mystical body of Christ." This, then, is a summary of the core meaning of what it means to be a Christian and what defines the Christian faith tradition. As any reader may infer, the cosmic picture painted by Christians is breathtaking- inspiring for many and appalling for others. Its breathtaking nature suggests clearly that to become a Christian or to call oneself a Christian is no small matter. It is, rather, a life-transforming matter, a radical reworking of life as ordinarily lived. It remains to be explained, then, why so many Christians, the vast majority it seems, are so ordinary.
A simple solution to this mystery has been, historically, to reserve the name "Christian" for only a few, that is, for those whose lives do indeed reflect the radical spiritual message found in the life of Jesus. However, making the judgment as to who is and who is not a Christian has, historically, been divisive, to say the least, so much so that some would rather adhere to the Biblical prescription to "judge not" and leave the answer to the question of who is a Christian up to God.