(19) So we see that they could not enter in because of unbelief.
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Paul puts his finger on the source of the Israelites' problem, why their heart could not be changed, why they consistently and persistently sinned and rebelled: "So we see that they could not enter in because of unbelief." Paul later turns this thought into an admonition for us:
Therefore, since a promise remains of entering His rest, let us fear lest any of you seem to have come short of it. For indeed the gospel was preached to us as well as to them; but the word which they heard did not profit them, not being mixed with faith in those who heard it. (Hebrews 4:1-2)
Not only did Israel have the witness of numerous demonstrations of God's presence and power among them to provide a foundation for faith, but they were also given the Word of God by His servants Moses and Aaron. In addition, they had living examples of faith in Moses, Aaron (most of the time), Joshua, Caleb, and others. God supplied these men with gifts by His Spirit as a testimony that should have provided more incentive for the Israelites to believe Him. But Hebrews 3:17 says He was angry with them forty years! If ever a people almost drove God to the point of exasperation, it was Israel in the wilderness.
We must not allow such a powerful lesson to pass by unheeded. Paul agrees, "For whatever things were written before were written for our learning, that we through the patience and comfort of the Scriptures might have hope" (Romans 15:4).
The lesson is clear. Those who believe God reveal their faith by obeying Him. Those who do not believe, disobey. Hebrews 3:12 warns, "Beware, brethren, lest there be in any of you an evil heart of unbelief in departing from the living God." Unbelief is evidence of an evil heart, and an evil heart departs from God. Like Hebrews 3:16—4:2, this verse equates unbelief with disobedience.
— John W. Ritenbaugh