I'll add this in for further understanding.
The same "belief" that someone believes with, is the same, but exact opposite, "disbelief" that someone disbelieves with.
This is where people get 'hung' up on the truth. Because some people think that it is not an ongoing belief in Christ, but a singular point in time, then they also think the same for disbelief in Christ. That is not the truth.
Disbelief is not a momentary 'not wanting to believe' in Christ. No more than Peters lapse in judgement was a total denial of Christ.
Disbelief that leads to foresaking Christ is a continual denial of Him. This comes through denial of His work in our lives. As James puts it;
Jas 1:12-15
Blessed is the man who remains steadfast under trial, for when he has stood the test he will receive the crown of life, which God has promised to those who love him. Let no one say when he is tempted, “I am being tempted by God,” for God cannot be tempted with evil, and he himself tempts no one. But each person is tempted when he is lured and enticed by his own desire. Then desire when it has conceived gives birth to sin, and sin when it is fully grown brings forth death.
Desire -----> Sin --------> Death.......
This is also why James then closes out his letter with this;
Jas 5:19-20
My brothers, if anyone among you wanders from the truth and someone brings him back, let him know that whoever brings back a sinner from his wandering will save his soul from death and will cover a multitude of sins.
We find ourselves wondering from the truth sometimes. I know I have at least. That did not mean that I have fully denied Christ though. However, had I continued on that path - or anyone else - that sin(which began with desire) would have eventually led to death. Not just physical death, but at James puts it - death of the soul.
Its not an - in/out - in/out - in/out - type of thing. However, when you let sin constantly live in your heart it will eventually harden it to the point of disbelief.
See, that's the point of watching ourselves about sin. Not continuing in sin is not about being good for God, its about guarding our hearts. Its not about earning salvation, its about nurturing it in our heart.