John Gill Commentary Hebrews 4:6

"Seeing therefore it remaineth that some should enter thereinto, and they to whom the good tidings were before preached failed to enter in because of disobedience," — Hebrews 4:6 (ASV)

Seeing therefore it remains
It follows by just consequence,

that some must enter therein ;
for God's swearing concerning some, that they should not enter into his rest, supposes that others should: and

they to whom it was first preached ;
to whom the Gospel was first preached, namely, the Israelites in the wilderness: entered not in because of unbelief; See (Hebrews 3:19) .