Adam Clarke on Hebrews 3:18

To whom sware he - God never acts by any kind of caprice; whenever he pours out his judgments, there are the most positive reasons to vindicate his conduct.

Those whose carcasses fell in the wilderness were they who had sinned. And those who did not enter into his rest were those who believed not. God is represented here as swearing that they should not enter in, in order to show the determinate nature of his purpose, the reason on which it was founded, and the height of the aggravation which occasioned it.