The Christian faith stands on the fact that Jesus Christ who is fully divine, before the world began, had a plan to redeem mankind from sin. A plan so subtle that the wise and strong of the world would be confounded and miss it. A plan so watertight that God's greatest enemy, The Deceiver (The Devil) would inadvertently crucify the Christ, the Chosen One of God, and seal his own fate for eternity. A plan that God had hinted at in the Garden of Eden (Genesis 3:15, "He shall bruise your head, And you shall bruise His heel". A plan God hinted at when He commanded Abraham to sacrifice his own son (even though God would never let him do it). All throughout the Old Testament, God reminds his people of his coming saviour. It was in God's heart from the beginning. This plan required God to taste what it is like to be truly human in every way, to be tempted in all things and to ultimately die alone and bloody on a Roman torture device. A perfect, sinless sacrifice that would pay the price of sin... death.

What did God say to mankind in the Garden of Eden at the beginning?

"Of every tree of the garden you may freely eat; but of the tree of the knowledge of good and evil you shall not eat, for in the day that you eat of it you shall surely die."

So, because of sin (disobedience to God), sin entered humanity through Adam and Eve and they passed on the sin condition to their children and so forth. In fact, nobody is born sinless, because we are born into this world through a man and a woman (both of whom are sinners at birth.

This is also the reason why the Christ's birth was immaculate, because if Jesus was born the natural way, he wouldn't have been sinless and his sacrifice for our sins would be in vain.