Verse 12 brings in another principle, which is that unbelief leads to departing from God. Throughout Hebrews, belief is basically synonymous with obedience, and unbelief or disbelief is interchangeable with disobedience. You see, what we truly believe will always show up in how we behave - in what we do.
The critical issue of belief isn't whether one believes that God exists, or even that Jesus died for sin. The Israelites had daily proof of God's existence, and the Hebrews were well-aware of Christ's sacrifice. The relevant question is whether God is believed - whether what He says is believed, and whether His character and nature, in their fullness, affect how we live. If we believe God, then His message - His word - becomes the ultimate guide for our lives. If we don't believe Him, we may call Him Lord and yet not do the things He says (Matthew 7:21-22).
