Albert Barnes Commentary Hebrews 3:18

"And to whom sware he that they should not enter into his rest, but to them that were disobedient?" — Hebrews 3:18 (ASV)

And to whom sware he. (See Barnes on Hebrews 3:11).

But to them that believed not. This refers to those who did not confide in God. Deuteronomy 1:32 states: Yet in this thing ye did not believe the Lord your God. In consequence of this want of faith, God solemnly swore to them that they should not enter the promised land. Deuteronomy 1:34–35 records: And the Lord heard the voice of your words, and was wroth, and sware saying, Surely there shall not one of these men of this evil generation see that good land, which I sware to give unto your fathers, save Caleb, etc.

The distinct reason, therefore, assigned by Moses, why they did not enter the promised land, was a want of faith; and this accords directly with the apostle's design here. He is exhorting those whom he addresses to beware of an evil heart of unbelief (Hebrews 3:12). He says that it was such a heart that excluded the Hebrews from the promised land. The same thing, he says, must exclude you from heaven—the promised home of the believer; and if that firm confidence in God and his promises which he requires is lacking, you will be excluded from the world of eternal rest.