Sin in the Garden
God had placed every type of tree in the garden to make it beautiful and give us a variety of food, and also had given us the tree of eternal life to eat and live forever in our present state. But the tree of the knowledge of good and evil was restricted, because in it we would acquire the knowledge to determine for ourselves the decisions we would make, rather than "becoming wise only by our obedience to God" (Calvin).
God wanted us to grow like we want our children to grow: through trust and obedience, rather than rebellion and self-sufficiency. Once they ate and their eyes were opened, the world was changed forever. No longer were they sure what was right, but their consciences were defiled and they covered their nakedness, and hid from God.
God banished them from Eden, and no longer were they able to access the tree of life. It would have to be God Himself who would bring us back through a garden (Gethsemane) to the tree of eternal life (the cross).