Faith is a gift of God, I agree. However, once given to us, it is ours to exercise or reject. If I gave you fifty dollars, it was mine until after I placed it in your hands, and then it becomes yours. A gift can also be rejected, or abused, IMHO.
Can you reject your living soul?
That is what God gave to Adam as a determiner to separate from all other matters of Creation.
The believer is a "new creature created in Christ Jesus" which separates the regenerate from the unregenerate, and that cannot be any more rejected than one can reject their living soul.
If God gives the gift, it is without repentance - that is man cannot "give it back."
Can it be abused? Certainly, and as Paul said, "that is why some of you are asleep (dead)."
There was an abuse of what the Lord established.
There are many Scriptures to warn against excess and abuse. "Do I sin that grace may abound? God forbid."
You did bring up a most important lesson.
What do I do with that "fifty dollars?"
Peter said, "add to your faith" and gave a series of stair step attributes that the believer is to attain.
Paul said, "be careful what you build upon that faith" and then talked of the judgment fire.
Jesus gave the parable of the servants each given an certain amount and each responsible for the care and growth of that amount.