What does the sin of unbelief mean in the Christian context?

Unbelief is a significant theme addressed in Hebrews, particularly the warning from Hebrews 3:12 regarding an evil heart of unbelief that departs from the living God. Scripture teaches that the core issue preventing Israel from entering God's rest was their unbelief. This refusal to believe is contrary to faith, which is essential for salvation and a relationship with God. As believers, we are cautioned to guard against this sin, recognizing that true faith involves trusting God's promises and relying on Christ's completed work for our salvation.