Now unto the King eternal, immortal (aphthartos), invisible, the only wise God, be honor and glory for ever and ever. Amen. (1Timothy 1:17, KJV).
The related word "immortality" is found five times in the KJV. It is derived from the Greek aphtharsia (Strong's #G861), as in Romans 2:7 or 2Timothy 1:10, or from athanasia (#G110), found in 1Corinthians 15:53 - 54, 1Timothy 6:16 and 2Timothy 1:10. Four times it is used in reference to a trait freely given to righteous resurrected humans and once to an intrinsic characteristic of God's existence (1Timothy 6:16).
On the one hand, to those who with patient endurance in good works are seeking glory and honor and immortality - eternal life (Romans 2:7).
For this corruptible (human flesh) must put on incorruptibility, and this mortal must put on immortality. Now when this corruptible shall have put on incorruptibility, and this mortal shall have put on immortality . . . (1Corinthians 15:53 - 54).
It is important to note that "immortal" or "immortality" are never used in relation to any part of human existence before conversion and resurrection. The teaching that all humans have an immortal soul, a spiritual substance that remains conscious after death, is not found in Scripture and is a false doctrine.
Scripture teaches that all humans are born completely mortal with no part of their existence being immortal. When a person dies, they lose consciousness and await a resurrection from the dead.
The righteous, at Jesus' return, will be raised and receive eternal life. The rest of humanity, who never had a full chance at salvation, will be brought back to life after the Millennium. It is at this time that they will receive their opportunity to understand the truth, repent, and ultimately live for eternity.