What happens to a born again person who does not Grow in Grace, and in Revelation Knowledge.
I have to answer it that way...for you.
In that case, they remain a "baby Christian", and will sin and confess, and live trying not to do that, until they die.
Its very likely they will become a cult member.
Its also likely that they will play with carnal behavior, and that will bite them one day with sickness, poverty, death, or similar, as you reap what you sow, eventually.
And God help anyone who listen to them teach the word of God.
That is like a 6 yr old, trying to teach Higher Mathmatics.
=Foolish.