Hi Aimiel,
I agree that God is immortal, "The King of kings and Lord of lords, who alone possesses immortality..." 1 Tim 6:15-16
However, this says that God ALONE is immortal, that means that souls are not naturally immortal, doesn't it? So souls are mortal.
Take a look at Matthew 10:28
“Do not fear those who kill the body but are unable to kill the soul; but rather fear Him who is able to destroy both soul and body in Gehenna." So the soul can be destroyed in Gehenna (Hell).
The soul is not immortal, and can be destroyed in Gehenna.
Now look at Ezekiel 18:4
"Behold, all souls are Mine; the soul of the father as well as the soul of the son is Mine. The soul who sins will die."
The soul is not immortal, God can destroy the soul, and the soul who sins will die.
The soul dies, it doesn't live forever, it dies as a result of sin.
Look at Romans 6:23
"For the wages of sin is death, but the gift of God is eternal life in Christ Jesus our Lord."
John 3:16
For God so loved the world that he gave his only begotten son that whosoever believes in him will not perish, but will have eternal life.
Since Jesus took our sins from us, we no longer owe the penalty for sin, which is death. Instead we can have eternal life. But this also means that whosoever rejects Jesus Christ will perish, and will not have eternal life.
The soul is not immortal, God can destroy the soul, the soul who sins will die, but whoever believes in Jesus Christ will not perish but will have eternal life.