God created wonderful things for man: the Sun, to give him light and warmth; the Moon, so that he would have brightness during the night; plants and fruit trees and animals to be at his service.
Adam and Eve were endowed by God with bodies of great beauty and comeliness; they possessed sanctifying grace, the gift of immortality and knowledge, and the gift of not suffering or enduring pain in this world.

Adam and Eve lived happy and content in paradise, loving God and loving each other. God gave man a command: “Of all the trees of paradise you may eat, but of the tree of the knowledge of good and evil you shall not eat, for on the day you eat of it you shall surely die.”

Envious of the happiness of our first parents, the devil wanted to make them sin so that they would lose the state of happiness and innocence they enjoyed in paradise. Taking the form of a serpent, the devil said to the woman: “Did God really command you not to eat from all the trees of paradise?” The woman answered the serpent: “We eat of the fruit of the trees of paradise, but of the fruit of the tree that is in the middle of paradise God said to us: Do not eat of it, nor even touch it, lest you die.” And the serpent said to the woman: “You will not die; for God knows that on the day you eat of it your eyes will be opened and you will be like God, knowing good and evil.” The woman looked at the fruit and saw that it was beautiful and good to eat; she stretched out her hand, took fruit from the tree and ate, and also gave some to her husband, who ate with her. In that instant the first sin in the world had just been committed. They were ashamed of being naked.

They heard God walking in the garden in the cool of the day, and Adam and Eve hid among the trees. But God called Adam and asked him: “Have you eaten from the tree from which I forbade you to eat?” And the man said: “The woman whom you gave me as a companion gave me some of it, and I ate.” Then God said to Eve: “Why have you done this?” And the woman answered: “The serpent deceived me, and I ate.”

God turned to the serpent and said to him: “I will put perpetual enmity between you and the woman, and between your offspring and hers. He will crush your head.” To Eve the Lord said that he would multiply her sufferings, and to Adam: “Because of you the ground will be cursed. By the sweat of your face you shall eat bread, until you return to the ground, for you are dust and to dust you shall return.” God expelled Adam and Eve from paradise. He placed before the garden a cherub with a flaming sword to guard the way to the tree of life. Our first parents left that marvelous place with hearts full of bitterness and sorrow. They had lost the happiness God had granted them.

Doctrinal explanation:

God created man so that in this life he might love and serve him, and afterward enjoy him eternally in heaven.
God endowed our first parents with every kind of good. He gave them a clear and luminous intelligence to understand all things, and a will inclined to the good. And above all, he gave them sanctifying grace, that divine gift which makes man an adopted child of God and heir of Heaven.
God placed Adam and Eve in a wonderful garden, full of delights. They were not subject to any evil; they had neither hunger nor thirst, neither sickness nor pain. They lived in peace, with great joy and happiness, having the immense joy of conversing with God in Paradise. And after this life God was going to transfer them to another place of greater happiness: Heaven.

But God disposed that, in order to go to Heaven, they had to become deserving of it; therefore the Lord gave them a command, saying: “Of all the trees of paradise you may eat, but of the tree of the knowledge of good and evil you shall not eat, for on the day you eat of it you shall surely die.” The devil, envious of the sweet friendship that existed between God and man, tempted them to eat of the forbidden fruit and said to them: “You will not die; for God knows that on the day you eat of it your eyes will be opened and you will be like God, knowing good and evil.” Adam and Eve ate of the forbidden fruit and sinned against the Lord. The instant they committed the sin, they were terrified and filled with shame over what they had done. This sin is called original sin.

Because of the sin committed, they ceased to be children of God; their soul became black as night, their passions were awakened, and they became subject to ignorance, sickness, and death. The gates of heaven were closed. All the goods with which God endowed our first parents, and which they were to transmit to all their descendants, were lost to them, and therefore we too were deprived of them. God so disposed it. Because of that sin, which is hereditary, all men are born without the life of grace.

God saw the pitiful state of man, had mercy on him, and promised him a Redeemer to repair the sin committed by our first parents and restore them to friendship with God.

Rule of conduct:

Original sin teaches us that God is the Lord who commands, and we are his children who must obey.

Source: Encuentra.com