The Theory is the Lie

"God had instructed the first humans that their lives depended on obedience to the rules of life established by Him as their Creator. God said “You may surely eat of every tree of the garden, but of the tree of the knowledge of good and evil you shall not eat, for in the day that you eat of it you shall surely die.” Genesis 2:16, 17 ESV. 

However, when Eve was confronted by Satan in the form of the

serpent, he questioned God’s direction, saying, “Did God actually say, ‘You shall not eat of any tree in the garden’?” 

Eve’s response was that God had told them to eat freely of all the trees of the garden except for this one tree. And if they ate, or even touched, the fruit, they would die. But Satan then replaced what God had said with a lie of his own, “You will not surely die.” Genesis 3:1–4 ESV

Satan’s lie became the beginning of the theory of unconditional human immortality.

God was no longer at the center of human life; instead, it was human ego. 

Convinced by the devil of their uniqueness and the greatness of their own “self,” humans believed that the ego could survive the death of the body, that it was something distinct from their physical tissue, which was subject to degradation and ultimately death, and therefore, that the ego is and must be immortal.

Death is described in the Bible as evil (Deuteronomy 30:15 ESV), a curse (Deuteronomy 30:19 ESV), bitterness (Ecclesiastes 7:26), terror (Psalm 55:4, 5), darkness and shadow (Psalm 107:10), and an enemy (1 Corinthians 15:26 ESV). 

Death, as the opposite of life, is portrayed as an adversary and something foreign to God."
StepsToLife