In regard to faith, we must understand what the Bible means by its frequent admonitions to "hear." Paul writes in Hebrews 3:15, "Today, if you will hear His voice." He is not pressing us to hear the sound of His voice, but to understand what God wants us to learn through what Paul, the preacher, is expounding in his epistle. Paul is urging us to take the time now to "get" it, to "see" or "grasp" what God is teaching.
Hebrews 3:17-18; 4:2 will help us reach a conclusion about what God intends regarding hearing. Whether a person physically hears the actual voice of God Himself is of little importance. Whether "hearing" in our personal reading or "hearing" the preaching of a minister, what is critical is that we obey the godly instruction, because unless we actually obey, we have not yet truly heard. If a person continues to sin, he has not really heard, in the biblical sense, what God has taught.
Put in another way, if a person continues to sin because God's Word does not motivate him to obedience to what He teaches, then he, in a worst-case scenario, either does not believe God or at this point his belief is so weak that he cannot bring himself to trust Him. Such are the ones who died in the wilderness. The weakness is not that people do not believe that He exists, but that they do not trust what He says because, in reality, they do not know Him. Thus, in the biblical sense, they have not yet truly heard.
In Hebrews 4:2, Paul uses the Greek word pistis for the first time in his letter. He will use it 31 more times. Pistis is translated either as "faith" or as "faithfulness." I believe that "faithfulness" is better here because that is what the Israelites lacked. Faithfulness is trusting God in continuous fashion as shown by conduct. God has given us a great deal, but it is our responsibility to hold firmly to His instructions by living them. Living them engrains them into our characters as habits, and this is good. Through habitual use, they become so entrenched in our behavior that we do not even have to call them to mind.
The unbelief that Paul is speaking of here is that our weak trust results in weak Christian living because we do not know and "see" God with the clarity that we should have. It can be rectified, but that is not always easy and at times may seem costly.