From Skin in the Game:
If humans were immortals, they would go extinct from an accident, or a gradual build-up of misfitness.
According to the Bible, this is, indeed, what happened. When humans could live almost 1000 years, we got so bad that God had to wipe the slate clean with the Flood. Something similar happened with the Tower of Babel. Lifespans had begun to shrink, but Shem still lived for 600 years and mankind had to be dispersed, or else the evil they would be able to accomplish would be considerable.
The question is often asked, “What could we do as a race if we could increase our lifespans by several orders of magnitude?” The accumulation of wisdom and experience would accelerate our development and allow us to do great things.
And yes, we would be able to do great things.
Great things that would be in service to greater and greater evil.
We are a fallen race, and while we are in these corruptible bodies, longer life would only exacerbate our corruption. What we need is a new type of life. What we need is new creation, not old creation stretched out and scraped thin.
Immortality would literally lead to our extinction. Banishment from the Garden was a punishment, yes, but like all discipline from God, it was also a grace.
“…lest he reach out his hand and take also of the tree of life and eat, and live forever…”
In The Magician’s Nephew, C.S. Lewis also hints at this truth. Jadis, the evil queen who would become the White Witch, takes fruit from a garden in Narnia, and gains everlasting life. And yet she is doomed to misery.
But length of days with an evil heart is only length of misery and already she begins to know. All get what they want; they do not always like it.Aslan
Without Christ, immortality is a curse. Thanks be to God for His mercy.