On the one hand:
...lies in your mistaken conviction that “the main core” of the Christian worldview is “the fall of man,” where the fall of man is apparently understood to imply, not only the doctrine of original sin, but also the origin of human disease and death as a result of human sinfulness.
This is a horribly distorted view of Christianity. Not even the doctrine of original sin is essential to Christianity, as the example of Eastern Orthodoxy plainly shows, since Orthodoxy does not accept the Catholic doctrine of original sin and yet is one of the major branches of Christendom. You protest, “If there was no fall of man, what sin is there to save us from?” That’s easy to answer: every man’s own sin. You hardly need to believe in the doctrine of original sin in order to recognize that all men have sinned and are therefore in need of God’s forgiveness. Indeed, this is the message that is emphasized throughout the Bible, not the doctrine of original sin.
Moreover, the idea that human physical death and disease is the result of sin or the fall, though championed by Young Earth Creationists, cannot be found in the biblical text and is widely rejected by many committed Christians (including me).
On the other hand:
21 For as by a man came death, by a man has come also the resurrection of the dead. 22 For as in Adam all die, so also in Christ shall all be made alive (1 Cor 15:21-22).
12 Therefore, just as sin came into the world through one man, and death through sin, and so death spread to all men because all sinned— 13 for sin indeed was in the world before the law was given, but sin is not counted where there is no law. 14 Yet death reigned from Adam to Moses, even over those whose sinning was not like the transgression of Adam, who was a type of the one who was to come…17 For if, because of one man's trespass, death reigned through that one man… (Rom 5:12-13,17).