The notion that Christianity is in fact a humanism, and the original and greatest humanism at that, has become of much of my own religious and philosophical thinking. I found this passage by Paul Tillich on St. Irenaeus of Lyons to do a particularly good job of explaining why this is so. [emphasis mine]
In Christ the new mankind has started. That which mankind was supposed to become, once disrupted by Adam, has finally reached its fulfillment in Christ. However, not only mankind but the whole cosmos finds its fulfillment in the appearance of the Christ. In order to accomplish this, Christ had to participate in the nature of Adam. Thus Christ is the beginning of the living as Adam is the beginning of the dead. Adam is fulfilled in Christ; this means that Christ is the essential man, the man Adam was to become but did not actually become. Adam was not in a state of fulfillment from the beginning; he lived in childish innocence. Here we have a profound doctrine of what I call a transcendent humanism, a humanism which says that Christ is the fulfillment of essential man, of the Adamic nature. Such a fulfillment became necessary because a break occurred in the development of man; Adam fell away from what he was to be come. The childish innocence of Adam has been lost; but the second Adam can become what he was to become, fully human. And we can become fully human through participation in this full humanity which has appeared in Christ. This includes eternal life, similitude with God with respect to participation in infinity.
Paul Tillich, A History of Christian Thought: From Its Judaic and Hellenistic Origins to Existentialism, p. 45