Wrong question.......Why in your doctrine does it state that people can loose, forsake or forfeit the gift of eternal life when scripture states the opposite.
There is no scripture that says once you are saved you are always saved.
There is a passage that says if your heart is right you will retain the word and persevere in fruitfulness.
That's the only kind of believer I see in scripture that can even begin to make a claim to being 'once saved always saved'.
Now back to my question:
Since people get new hearts when they get saved, why do people stop believing according to your doctrine?
Don't get me wrong, I agree that believers get new hearts, and believers really can and sometimes do stop believing. But you were trying to use the 'new heart' argument to suggest every believer has the 4th type of soil in the Parable of the Sower and, therefore, all believers persevere to the end and never lose salvation. But you must have forgotten that your doctrine says a person can stop believing.
The believer is “in Christ.”
And as a result of being “IN Christ,” you have eternal life (1 John 5:11; 2 Cor. 5:17).
If you ride in a car that runs forever, does 'forever' mean you will always be in the car no matter what?
No, of course not. It means the car runs forever whether you're in it or not.
Hebrews explains how Christ and the life he gives is eternal and everlasting, not your possession of it.
You won't possess Christ's eternal life without being able to lose it until you make it all the way to the end of this life in unbroken faith and enter into the kingdom.