Well, to me, there are different kinds of unbelief. For example, there is the believer who loses a friend or family member to murder or accident. They have believed in God, but now their child is gone and so they wonder "how could God let this happen" because they can't fathom how a loving God could allow such a heinous tragedy. Their mind and faith simply can't handle it so they lose (misplace) faith for awhile, until they're clear headed enough to look at the situation and see what good God has made from it.
Then there are the ones who believe in God, but have fallen so far away that they can't see or hear Him, and they feel He has deserted them. Then the devil creeps in and whispers "you're not worth it. God doesn't love. He doesn't care if you live or die, you rotten piece of scum".. These are the people who listen and take heed to what satan tells them, and they stop trusting and believing in God altogether..
The difference is, in the first example, the person who lost a child to murder, was able to later go back and assess the situation and see that God brought something positive from it. Maybe their child was an organ donor, and there was a person in hospital who desperately needed one of the deceased's vital organs. Yes, the parent would be devastated that their child died, but wouldn't they also be glad that their child's death wasn't in vain, particularly if it saved another person's life?
In my second example, that person is just so totally lost and alone and hurt. They have no protection from the devil. What he says makes sense to them. If God loves them, why did he allow (insert situation) to happen and why isn't He helping and where is He? That is the person who admits defeat to the devil, and doesn't care enough to fight to regain his or her belief in God. So they remain in that pity bog with the devil forever and in turn, forsake their belief, faith and Christian status forever.