"And the Lord God commanded the man, 'You are free to eat from any tree in the garden; but you must not eat from the tree of knowledge of good and evil, for when you eat of it you will surely die" (Genesis 2:16-17).
The Fall of man is an essential doctrine of historic Christianity. By Adam's one act sin entered the world, and through Adam all of humanity has inherited Adam's sin nature.
The tree of the knowledge of good and evil was a testimony of God's sovereignty over all creation. Though Adam was allowed to eat from any tree in the garden, he freely chose to violate God's ordinance and do the one thing he was commanded not to do. Adam did this of his own free will; he was not coerced: "The woman You put here with me -- she gave me some fruit from the tree, and I ate it" (Genesis 3:12). The fruit was not something that had magical or mystical properties, but rather, was a simple means of demonstrating whether or not man would choose to love and obey God, or choose to reject God's sovereignty.
In the day that Adam and Eve ate the fruit they died spiritually. The close fellowship Adam and Eve had with God was broken. In time they would both, as God had promised, experience physical death as well. This would apply not only to Adam and Eve, but to all their descendants.
With the Fall came also a way of redemption. An animal's life was sacrificed to cover Adam and Eve (Genesis 3:21). This was a picture of the ultimate sacrifice that would occur in the distant future on Calvary. God, by his grace, would one day purchase man's redemption by paying the penalty for man's sin Himself.
In the generations to come the descendants of Adam and Eve, those who love God and are called according to His purpose, would be restored to fellowship through Jesus Christ. One day they will enter into God's kingdom and enjoy His fellowship face to face. Those who do not love God, and prefer sin instead, would reject the sacrifice. Ultimately, they will have to pay the penalty for their own sins, and be forever separated from the grace of God -- the God they have willingly rejected.
"But the gift is not like the trespass. For if the many died by the trespass of the one man, how much more did God's grace and the gift that came by the grace of the one man, Jesus Christ, overflow to many!" (Romans 5:15)