Just to play a devils advocate. Do these verses declare that unbelief is a sin that is not paid for by Jesus?
Hebrews 3:12-19
Take care, brothers, lest there be in any of you an evil, unbelieving heart, leading you to fall away from the living God. But exhort one another every day, as long as it is called “today,” that none of you may be hardened by the deceitfulness of sin. For we have come to share in Christ, if indeed we hold our original confidence firm to the end. As it is said, “Today, if you hear his voice, do not harden your hearts as in the rebellion.” For who were those who heard and yet rebelled? Was it not all those who left Egypt led by Moses? And with whom was he provoked for forty years? Was it not with those who sinned, whose bodies fell in the wilderness? And to whom did he swear that they would not enter his rest, but to those who were disobedient? So we see that they were unable to enter because of unbelief.
Hebrews 6:4-8
For it is impossible, in the case of those who have once been enlightened, who have tasted the heavenly gift, and have shared in the Holy Spirit, and have tasted the goodness of the word of God and the powers of the age to come, and then have fallen away, to restore them again to repentance, since they are crucifying once again the Son of God to their own harm and holding him up to contempt. For land that has drunk the rain that often falls on it, and produces a crop useful to those for whose sake it is cultivated, receives a blessing from God. But if it bears thorns and thistles, it is worthless and near to being cursed, and its end is to be burned.
I do not think these verses teach that God never died for unbelief in a Christian.
I do think these verses support a limited atonement whereby a redeemed Christian will believe because the atonement of Christ, for the elect of God is effective to secure a perseverance in belief.
Those who fall away were/are types of Judas Iscariot who hid his blasphemy against Christ until the betrayal.