Boiled down it's really very simple. It is by faith we are saved,but that
faith needs to be cared for otherwise it dies. It the same manner as a plant,it needs to be feed,watered and protected otherwise it dies. A dead faith can not save us.
But isn't faith the gift (Php 1:29; 2Pe 1:1; Ac 18:27; Ro 12:3) which accompanies rebirth?
And rebirth is the transformation (
metamorphoo) from spiritual death (no Holy Spirit life) to eternal life (Jn 5:24, 6:54) by grace through the gift of faith,
transformation from an enemy of God to an adopted child of God,
as the transformation (metamorphosis) from a caterpillar to a butterfly.
And, as in the natural order, one can't be unborn after they are born,
or cease being an adopted child, for they have the same legal status as a natural child, neither being changeable,
nor can a butterfly return to being a caterpillar.
The gift of faith, which is true faith, does not return to unbelief, anymore than a butterfly returns to being a caterpillar.
What "dies" is counterfeit faith, which had no root of rebirth and falls away in the time of testing (Lk 8:13).
Apostasy from one's profession of faith shows one's faith was counterfeit (1Jn 2:19).
God's saving gifts of rebirth, faith, repentance, justification/righteousness are irrevocable.
And it is God who uses many ways, including exhortations and warnings, chastisements and trials, to see to it that they are nourished and do grow.
But God's irrevocable gifts cannot "die" and return to unbelief, as the butterfly cannot return to its former state.