God placed Adam in the The Garden of Eden and gave him Eve, whom He created from Adam's rib, to be a companion for him.
God gave them a commandment that they may eat of all the fruit of the trees except of the The Tree of Knowledge of Good and Evil, and on the day they eat of it they would surely die.
Many speculate what the forbidden fruit might have been, but in truth it doesn't matter whether it was an apple or orange.
The reason for the commandment was not to deprive them of the fruits of paradise, but to give them the opportunity to exercise their Free Will, either to follow God's will or to reject it.
It was a simple command, which gave them the opportunity to practice and advance in obedience, virtue and sanctity, an opportunity to gain the much-desired experiential knowledge.
The Devil, appearing in the guise of a serpent told Eve that if they eat of the tree "ye shall not surely die. For God knew that in whatever day ye should eat of it your eyes would be opened, and ye would be as gods, knowing good and evil."
(Genesis. 3: 4-5)
Eve was guileless and innocent, and she didn't immediately recognise that the serpent, the most cunning of the beats on the earth, was evil. She struck up a conversation with the serpent and with great trust she listened to the Evil one slander God and allowed herself to be led to the precipice of evil.
Indeed Eve was as Saint John Chrysostom says, "puffed up with the hope of becoming equal to God and imagined great things for herself".
Thus the tree, which she had seen many times before and only identified it with God's command, suddenly looked different. She looked upon it as for the first time and saw that the tree was good for food and it was pleasant for the eyes to look upon and beautiful to contemplate.
Believing the serpent, she ate of the fruit and gave it to Adam also with her, and they ate.
At first glance one might say their intention was good because their one desire was to reach perfection, but they sinned because they disobeyed God's commandment. They freely chose not to follow god's will and this destroyed or distorted in them the likeness of God.
The significance of their action goes much deeper, doing what the seducer suggested, man appeared to be saying to God: I have no need of you. I shall live by myself, self-sufficient and independent. I don't need your guidance or protection. I'm able by myself to live and achieve great things.
Indeed man's Original Sin revealed his unbelief in God, his egoistic rebellion against the Divine Majesty, his thanklessness and ingratitude toward the beneficent Creator and Father, his contempt, insult and blasphemy against the Holy and Heavenly King.
Because Adam did not conceive and discover evil by himself, but was tempted from without by the Devil and thus led into Sin, God gave them the opportunity to Repent and ask for forgiveness.
He first approached Adam and asked what he had done, who replied: "The woman whom Thou gavest to be with me, she gave me of the tree and I did eat" (Genesis. 3: 12)
Adam did not accept that he had made a mistake, but passed the blame to God for giving him the woman.
God then approached Eve and she replied: "The serpent beguiled me and I did eat".
(Genesis. 3: 13)
Like Adam, Eve was too proud to accept her mistake and ask for forgiveness and passed on the blame onto someone else, in this case the serpent.
The first Sin was disobedience to God's will and this immediately produced an offspring in Pride, and subsequently a long chain of other sins.
The fact that God gave them the opportunity to Repent does not necessarily mean that if they had taken this path they would have regained the likeness of God. God never lies; He said that on the day they eat of the Tree of Knowledge of Good and Evil, they would surely die.
This did not mean that thy would immediately die a physical death. Eternal life can only be lived as it is lived in the likeness of God. They freely chose to separate themselves from God's likeness by following their own Free Will. This separation was therefore death as opposed to immortal life.