I am a saved, born again Christian.
The saved, Spirit sealed Galatians stopped believing in the sufficiency of Christ for justification and went back to observance of the old covenant Festival cycle and to a different gospel. And some of the saved Corinthians stopped believing in a risen Christ. And we know it was saved people who did that because Paul said their faith has been in vain if it's true that Christ has not risen from the dead. They started believing in a different Christ, a different gospel than Paul had preached to them and which they received by which they were saved.
But if you want to believe you'll never stop believing, that's fine. I'm not going to take that away from you. You may very well never stop believing. But to project your experience onto other believers would be a mistake. Not all believers are as mature as you might be. Jesus gave various reasons why some people do not retain the word of God in them. Which, according to the Parable of the Sower, is likely to happen in those in whom the word of God is not deeply entrenched.
Paul talks about 'carnal' Christians here.......
I, brethren, could not speak unto you as unto spiritual, but as unto
carnal,
even as unto
babes in Christ.
2I have fed you with milk, and not with meat: for hitherto ye were not able
to bear it, neither yet now are ye able.
3For
ye are yet carnal: for whereas
there is among you envying, and strife, and divisions,
are ye not carnal,
and walk as men?
4For while one saith, I am of Paul; and another, I
am of Apollos;
are ye not carnal? 1 Corinthians 3:1-4