Hebrews 3:16-19 (ESV)
For who were those who heard and yet rebelled? Was it not all those who left Egypt led by Moses?  And with whom was he provoked for forty years? Was it not with those who sinned, whose bodies fell in the wilderness?  And to whom did he swear that they would not enter his rest, but to those who were disobedient?  So we see that they were unable to enter because of unbelief.
A very large crowd of Israelites left Egypt with Moses and crossed the Red Sea and saw the miracles in the wilderness and ate manna and quail, and so much more. Only two of them made it into the Promised Land. The reason they did not enter the land was because they were disobedient. The rebelled against God. All the blessings they received from God did not mean that they were the people of God in any spiritual sense.
The people of God are those who believe and obey. We see this in verses 18 and 19. God swore that the disobedient would not enter the rest that awaited those who would enter the land. So why did they not enter? Verse 19 tells us. Because they did not believe. They did not believe the threat that God had made. They didn’t believe the promises that God made to them. The proof that they did not believe? They went right ahead and disobeyed the commandments.
Real belief will produce obedience. Their lack of belief however was more than just not believing the threats and promises that God made. It was a matter of not entrusting themselves to God. They did not have faith in God. They knew better then God. They had a better idea. The believed the lie that God’s way was not best.
This is very relevant for us today. The writer to the Hebrews is using the people of the Exodus to warn the people that he is writing. If you are not careful you will have happen to you what happened to them. The church is full of people who say they believe but who do not live in obedience to the commands of God. They live in security thinking that God will never refuse them entrance into eternal life because they made some profession of faith or “believe” in Jesus. But belief that does not translate into godliness is not faith in Christ. A faith that does not believe that God’s way is always best and that He knows what He is doing is not faith at all.
People with such “faith” will not enter God’s rest because they do not believe. They will not enter God’s rest because they are disobedient. This is not two things. It is one thing. Faith in Christ is marked by obedience. Obedience is the proof of faith. If we say that people are saved by obeying the commands of God we risk falling into the Gospel denying doctrine of salvation by works. We must always get the order right. Faith produces obedience. Faith in God that is not accompanied by works is an impossibility. Works without faith is every religion in the world except the Gospel of Jesus Christ.