Whoever believes in him is not condemned, but whoever does not believe stands condemned already because he has not believed in the name of God’s one and only Son. (John 3:18)Unbelief is a great sin. It applies to those who have heard the gospel and not believed, those who have not heard the gospel, and those who heard, believed, and in a believing state do not believe God.
Whoever believes in the Son has eternal life, but whoever rejects the Son will not see life, for God’s wrath remains on him. (John 3:36)
See to it, brothers, that none of you has a sinful, unbelieving heart that turns away from the living God. (Heb. 3:12)
Many real Christians fall into this last category. It is possible to understand unbelief in those who have not heard (even though is it still inexcusable—cf. Romans 1:20), because it is the gospel that causes belief. It is very difficult to understand unbelief in believers.
You foolish Galatians! Who has bewitched you? Before your very eyes Jesus Christ was clearly portrayed as crucified. I would like to learn just one thing from you: Did you receive the Spirit by observing the law, or by believing what you heard? Are you so foolish? After beginning with the Spirit, are you now trying to attain your goal by human effort? Have you suffered so much for nothing-- if it really was for nothing? Does God give you his Spirit and work miracles among you because you observe the law, or because you believe what you heard? (Gal. 3:1-5)These Christians did not remember how they were saved; they had forgotten what they believed. Another explanation is that Christians are not reading their Bible. Consequently, the world is their major source of belief. It is a bad source.