I apparently did not explain the whole “sin of unbelief” thing well at all. Let me try again!
There is the sin of complete unbelief – that is not believing in or trusting Jesus as Savior and Lord – that is an unforgivable sin if we remain unbelieving and die in that state. This would be an unbeliever. We are all unbelievers to start with.
Thankfully, praise God, our unbelief is forgivable by God if we repent of our unbelief and sins and turn to Christ in total faith, receiving Him as our Savior and LORD in this lifetime, yielding ourselves fully to Him. But if we remain in our unbelief in Christ and we reject Him, then this sin cannot later be forgiven after death.
Believers may struggle and wrestle with weak faith or doubts. That is not what I am talking about here. I went into more detail on this in a post a few days ago.
If you missed my earlier post, you are welcome to check it out.