Adam and Eve were created to commune with God and attain theosis (salvation). To go against what they were told by their Creator was a choice they had and did make- the consequence being that death entered the world.
Christ was prophesied to die on the cross because it was cursed due to the sin of Adam and Eve. Christ descended into hades and defeated death and sin. That was the entire purpose for Christ dying on the cross. This idea that He was both fully God and fully man gave people a way to be reconciled to God.
Interestingly, it is a pious opinion in Orthodoxy that even if Adam and Eve had not sinned, and death had not entered the world- that Christ still would have come in the flesh in order to help us attain theosis (salvation). "God became man, so that man can become like God." Plus, it wasn't too late for Adam and Eve- even then. In the icon of the Resurrection- Christ is depicted trampling the gates of Hades and lifting Adam and Eve from their graves. Christ destroyed death. (It is even mentioned in the New Testament how after the Resurrection- many of the righteous dead were resurrected and appeared to many people.) Like Adam, we are dead in our sins, but through Christ (the New Adam) we are brought to life (where we were once dead in our sins).
Anyway, the 'tree of life' is seen as a prefiguration of the Cross, which we couldn't partake of until after the resurrection of Christ, as evidenced in the hymnography of the Church:
Make ready, O Bethlehem, for Eden hath been opened for all. Prepare, O Ephratha, for the tree of life hath blossomed forth in the cave from the Virgin; for her womb did appear as a spiritual paradise in which is planted the divine Plant, whereof eating we shall live and not die as did Adam. Christ shall be born, raising the image that fell of old.
Prophecy of Isaiah 60:11-16:
Therefore your gates shall be open continually;
They shall not be shut day or night,
That men may bring to you the wealth of the Gentiles,
And their kings in procession.
For the nation and kingdom which will not serve you shall perish,
And those nations shall be utterly ruined.
"The glory of Lebanon shall come to you,
The cypress, the pine, and the box tree together,
To beautify the place of My sanctuary;
And I will make the place of My feet glorious.
Also the sons of those who afflicted you
Shall come bowing to you,
And all those who despised you shall fall prostrate at the soles of your feet;
And they shall call you The City of the LORD,
Zion of the Holy One of Israel.
"Whereas you have been forsaken and hated,
So that no one went through you,
I will make you an eternal excellence,
A joy of many generations.
You shall drink the milk of the Gentiles,
And milk the breast of kings;
You shall know that I, the LORD, am your Savior
And your Redeemer, the Mighty One of Jacob.
I Corinthians 15: 54-58:
So when this corruptible shall have put on incorruption, and this mortal shall have put on immortality, then shall be brought to pass the saying that is written, Death is swallowed up in victory. O death, where is thy sting? O grave, where is thy victory?
The sting of death is sin; and the strength of sin is the law.
But thanks be to God, which giveth us the victory through our Lord Jesus Christ.
Therefore, my beloved brethren, be ye stedfast, unmoveable, always abounding in the work of the Lord, forasmuch as ye know that your labour is not in vain in the Lord.
From the Paschal Homily of St. John Chrysostom:
Let no one bewail his poverty,
For the universal Kingdom has been revealed.
Let no one weep for his iniquities,
For pardon has shown forth from the grave.
Let no one fear death,
For the Saviour's death has set us free.
He that was held prisoner of it has annihilated it.
By descending into Hell, He made Hell captive.
He embittered it when it tasted of His flesh.
And Isaiah, foretelling this, did cry:
Hell, said he, was embittered
When it encountered Thee in the lower regions.
It was embittered, for it was abolished.
It was embittered, for it was mocked.
It was embittered, for it was slain.
It was embittered, for it was overthrown.
It was embittered, for it was fettered in chains.
It took a body, and met God face to face.
It took earth, and encountered Heaven.
It took that which was seen, and fell upon the unseen.
O Death, where is thy sting?
O Hell, where is thy victory?
Christ is risen, and thou art overthrown!
Christ is risen, and the demons are fallen!
Christ is risen, and the angels rejoice!
Christ is risen, and life reigns!
Christ is risen, and not one dead remains in the grave.
For Christ, being risen from the dead,
Is become the first-fruits of those who have fallen asleep.
To Him be glory and dominion
Unto ages of ages.
Amen.