John Gill Commentary Hebrews 3:19

"And we see that they were not able to enter in because of unbelief." — Hebrews 3:19 (ASV)

So we see that they could not enter in
To God's rest, the land of Canaan, for they died by the plague before the Lord, and their carcasses fell in the wilderness, before they came to it, (Numbers 14:37Numbers 14:45) and the reason was,

because of unbelief ;
their distrust of God, his power, and his providence; this instance is produced by the apostle, to show the evil nature of unbelief, and the sad effects of it; to deter persons from it, and that they might take heed of encouraging it; and which instance he further improves in the following chapter.