The “therefore” points us forward. Based upon being reminded of those who had hardened their hearts against God even in the midst of being shown true things, these Jewish Christians need to be careful. Not listening, rejecting what God was doing is a risk that is just as present for these folks today as it was for their ancestors. Also, notice the quotations, the author continues the exegesis of Psalm 95, explaining how it applies to the generation that is being spoken to.
The subsequent discussion about “rest” appears to combat any restrictions that may have been anticipated on what God’s rest is. Perhaps they thought it was the promised land, but the writer here makes it clear God’s rest is still an appropriate aspiration today as it was when David wrote of it in Psalm 95 (obviously after the time of Joshua and the promised land). How should we think of it? I’m banking on the ultimate return of Jesus and final establishment of the Kingdom.
So back to the encouragement, the Hebrews are to strive to enter that rest. But just as in the past, God will not be fooled. His “word” is living and active, sharper than any double-edged sword. (Word here really can’t mean Scripture, although the way folks use it to refer to Scripture isn’t untrue, either, it just isn’t particularly correct in this context.) “Word” here probably is safe as the “action” or “expression” of God, which is still living and active and it is that which cuts to the root of things, gets to them as they are. Again, there’s no fooling God, any impression otherwise is frankly ridiculous.
And yet, again, we have Jesus. Jesus the High Priest, aware of our frailties, sympathetic in our weaknesses. And the thing we do not deserve, confidence at the throne of God, we are graciously and mercifully given. That, my good fellas, is beautiful.