Ah the ol' No True Scotsman Fallacy.
Even if true that someone who ends up in hell was no true Christian to begin with, it's no comfort to the believer because every Christian who ends up in hell thought he was a Christian. What keeps any of us from believing the same thing, that we are true Christians, and end up being the ones cast into outer darkness? I call this Eternal Insecurity, one of the products of Calvinism - for those who can see it.
Hi again! God invites us to be assured of our salvation and gives strong promises to us. We rest only in Christ to be saved, and cannot trust anything else. John 10:28-29
"I give them eternal life, and they shall never perish; no one will snatch them out of my hand. 29 My Father, who has given them to me, is greater than all[a]; no one can snatch them out of my Father’s hand."
What about people who either don't trust in Christ or haven't heard of him? What happens to them after they die?