8 For if Joshua had given them rest, then He would not afterward have spoken of another day.
The “for” here is the basis for verse 7.
if Joshua had given them rest,
Joshua became the chief leader of Israel after the death of Moses (Josh 1:1-2). The Greek indicates that the “if” here is a contrary to fact condition (second class condition): Joshua did not give Israel rest in his day.
then He would not afterward have spoken of another day.
God did indeed speak of “another day.” If Joshua had given Israel rest, then God would not have spoken of another day.
The “rest” that Joshua gave when Israel settled in the Promised Land was not God’s final or ultimate rest for the readers of Hebrews. There remained a promise for believers of the church age. The conquest of Joshua’s day did not conclude God’s offer of rest. Psalm 95:11 proves otherwise.
There remains a rest for believers today.
In Joshua’s day Israel had to fight to possess the land. They failed to fully accomplish their task. What Jesus did far exceeded what Joshua accomplished. What is available for believers today exceeds what Israel had.