We have already considered the verb that expressed unbelief (apisteo), and the noun apistia is obviously related
Jesus did not do many miracles in His hometown because of their unbelief (Matt. 13:58; Mk. 6:6). The disciples were unable to cast out a particularly stubborn demon because of their unbelief (Matt. 17:20). It is striking that in Mark’s account of that same incident the father of the demon-possessed boy cried out for Jesus to save him from his own unbelief (Mk. 9:24) Jesus rebuked His disciples after the resurrection for their reluctance to believe those who had reported Him risen (Mark 16:14).
Paul exults in the fact that the unbelief of man is incapable of thwarting the faith of God (Rom. 3:3). When Abraham was promised a son in his old age, he did not stagger in unbelief (Rom. 4:20). The unbelieving Jews were broken off the olive tree of the true Israel because of unbelief (Rom. 11:20), and if they give up that unbelief, they will be grafted back in again (Rom. 11:23). Before Paul was converted, he was an evil man, and like his countrymen was caught in the web of unbelief (1 Tim. 1:13).
Christians are told to guard against the sin of unbelief (Heb. 3:12), a sin that results in departing from the living God. The Jews in the wilderness could not enter into God’s rest because of unbelief (Heb. 3:19). In the same way today, Christians are to resist unbelief, a sin that remains a sin even if we give it a fancy theological name.