THEY COULD NOT ENTER
"So we see that they could not enter in because of unbelief." (Hebrews 3:19)
Even though "with God all things are possible" (Matt 19:26), there are things He cannot do. For example, God "cannot lie" (Tit 1:2), and "cannot deny Himself" (Tit 1:2). He "cannot be tempted with evil" (James 1:13). There were environments in which Jesus, in whom the "fullness of the Godhead" dwelt "bodily" (Col 2:9), could "do no mighty work" (Mk 6:5).
As well, there are things that men cannot do – things God's nature will not allow them to do. Our text is a case in point. God had brought the Israelites out of Egyptian bondage, just as He promised Abraham He would do (Gen 15:14; Deut 4:20). He led them through the Red Sea, parting it for them, and bringing it together again upon their enemies (Heb 11:29). He fed them with manna, and gave them a miraculous supply of water as they journeyed through the wilderness (1 Cor 10:3-4). If anyone could be locked into Divine favor, surely it was this people. They had the favor of God, the promise of God, the guidance of God, and the provision of God. A land was prepared for them, and they were told they would enter into it.
However, "with many of them God was not well pleased: for they were overthrown in the wilderness" (1 Cor 10:5). Their cries in Egypt had been heard, and their oppression was seen  (Ex 3:9). They were led out of bondage, and given favor in the sight of the Egyptians. They crossed an impassable sea, ate miraculous bread, and drank supernatural water. They were not sick, and their feet did not swell. Yet it says of those who fell in the wilderness, "they could not enter in because of unbelief." Their refusal to trust God excluded them from the land for which they were delivered. When they ceased to rely upon the Lord, the door to the promised land was shut for them. When they no longer leaned upon the God of heaven, He barred them from Canaan.
There is a significant number of people – professed believers – who think they can get to heaven without trusting God, believing Him, or having faith in Him. They have chosen to love and rely upon other things, yet imagine that some allegiance to the Lord in the past will carry them into the presence of God where they will be accepted by Him. This is a delusion, and there is not a grain of truth in it. Believing is what puts us into the favor of God and prepares us for entrance into glory. Unbelief cannot obtain the promises of God, nor is God committed to do anything but condemn those who are ruled by it. Just as it was with Israel, we will find in the last day that many "church members" will not be able to enter into glory because of unbelief.