Unbelief of the unregenerate stems from the arrogant rejection of God's drawing of a man.
Which leads to apathy towards those who accepted God's drawing.
Now... that is the unbelief of those who reject salvation.
Unbelief hardens over time and won't know the real you standing there.
They can only see your body, but they see not the soul.
There is also unbeliever of the regenetate type.
They set limits with God and how much God is to manifest in their thinking and actions.
They find their "God limiter" according to what inaccurate doctrine they find an affinity for.
We know unbelief also applies to the believer. We can know by the fact of the verses
you cited to warn the regenerate not to fall into unbelief.
What I was trying to get at was this.
We need to get away from pat answers and to delve into the attitude of an unbeliever possesses.
And.. why one rejects sound doctrine and clings to their pet false doctrines.
It's easy to not define, and simply cite verses that serve a purpose of settling an issue as a command only,
but, without understanding.
Proverbs 4:7
The beginning of wisdom is this: Get wisdom.
Though it cost all you have, get understanding.
We have been commanded to get understanding to be added to our knowledge of the Word.
Knowledge of the Word is only step One.
Proverbs 1:7
The fear of the Lord is the beginning of knowledge,
but fools despise wisdom and instruction.
Some only want knowledge of verses as a means to remain robotically obedient...
They know they should smile and be nice.. And, that's about all.
Where's the life Jesus wants us to have?
That only comes from when gaining understanding.
Not simply repeating verses that have yet to be analyzed by someone with the gift and be understood.
John 10:10
The thief comes only to steal and kill and destroy;
I have come that they may have life, and have it to the full.
Satan must let Christians cite Bible verses. He can't avoid it.
Its understanding that he fights them from gaining.
You can have a million Bible verses memorized, yet have no understanding.
And, think you are a great Christian (in a humble sort of way, of course).
So be it... Grace and peace.