Hebrews 3:16-19
Heb 3:16 (KJB)
For some, when they had heard, did provoke: howbeit not all that came out of Egypt by Moses.
Provoke - Rebelled
Here the writer is teaching us that those who came out of Egypt under the leadership of Moses had rebelled against God. The only two of the original group from Egypt were Caleb and Joshua who believed God. All the rest had perished in the wilderness because of their unbelief and murmuring. We are being taught here how dangerous murmuring and rebelling against God can be, even for a believer.
Heb 3:17 (KJB)
But with whom was he grieved forty years? was it not with them that had sinned, whose carcases fell in the wilderness?
Was he grieved - Be angry at or displeased
Carcases - Dead body or corpse
The evidence for the sinning attitude of Israel is strewn throughout the first five books of the Bible. The Pentateuch reveals to us that throughout the wilderness wandering the rebellion of the Israelites was always present and that is why God had struck them. They were delivered from slavery in Egypt and their sinful rebellion erupted almost immediately upon leaving Egypt. That rebellion did not stop until the last one was judged by God and destroyed and then they went into the promised land under the leadership of Joshua.
Heb 3:18 (KJB)
And to whom sware he that they should not enter into his rest, but to them that believed not?
This verse is a question literally detailing the very sin which underlies all rebellion against God and that is unbelief. The writer did not have to go into detail concerning the way unbelief manifests itself because we see throughout the pages of the Pentateuch what they are, such as rebellion, murmuring, defiance, complaining, etc. Unbelief is at the root of all these things. When a person becomes a true believer, they rest in the Lord. We can see the evidences of unbelief today. Those who have no belief in God will create systems such as evolution which denies creation. Church works programs which circumvent the cross. Intellectualism which elevates man and reduces God to a “concept.” All these unbelievers have no rest. The rest of God is His salvation. Once a person becomes saved, they no longer seek the wisdom of the world and the ways of the world. A true Christian rests in Calvary. A religious Christian, which is a false Christians, rests in the ways of the world and has no rest because, either you are resting in Calvary or you are in the world.
Heb 3:19 (KJB)
So we see that they could not enter in because of unbelief.
The summary is very straightforward in that the reason the Israelites did not enter into the promised land was because of unbelief. They refused to take God at His word and sought their own ways. Their own ways had gotten them into trouble and hardened their hearts toward God.