How do we know that unbelief is a sin?

Unbelief is recognized as the fundamental sin that alienates humanity from God. Jesus refers to unbelief as sin in John 16:9, where He explains that the Holy Spirit will convict, specifically about not believing in Him. Additionally, Scripture shows that unbelief not only prevents individuals from accepting the gospel but also leads to their condemnation (John 3:18). The idea that one can be excused for lack of faith is a misunderstanding of the nature of God and His clear call for belief. Through countless biblical examples, such as the Israelites' failure to enter the Promised Land due to their unbelief (Hebrews 3:19), we see that continued doubt is an affront to God's justice and grace.