That has been debated forever almost and it's a very good question, seems important.
I *think* you might be talking about two things. Can a Christian lose his salvation and can a believer turn his back on God and walk away from Him...and still be saved?
I don't think that anyone can be rejected by God if they have a repentant heart and believe in Him and what He did or us.
But there are a few scriptures that will make one raise his eyebrows.
Revelation 3:5,
Psalm 69:28
That verse from Revelation sounds like a warning to me. Can a person get saved and accept the Lord but then go home and sit on his blessed assurance? Not join a church. not read the Word like he should or even pray, and still be saved? Or as some would say, these were never really saved at all...? But if that's so then...we don't receive salvation when we pray for it but receive it later, after showing due diligence? I dunno. Not a simple question to answer for sure. I guess that's where the fear and trembling come in to play.