Because God has spoken to us by His Son, and because His Son is so great and so glorious, and because the subject which is addressed is of such infinite importance to us and to our welfare, He says we ought to give the more earnest heed to it.
Earnest is an important word. It means "abundantly," "more exceedingly," "much more frequently," or "more super-abundant" heed. Paul is saying to pay attention intensely to what God is doing in our lives!
We should pray and study with great care and concern lest we should let God's Word slip, which means to "let it [God's Word] run out"—to leak out like a barrel with a cracked plug. The barrel is full, and it very slowly starts to leak.
Another analogy would be to "drift away." Envision a rowboat tied to a pier, but the rope loosens and falls into the water. Someone on hand could reach down, grab the rope, and retie it. But if this simple task is neglected, then the boat, which had been floating right next to the piling, slowly drifts away. Soon it will be ten feet away, then fifty feet, and in time it is on the horizon where the water is rough. Paul instructs us not to let that happen. Do not let it drift away! Pay attention! If we become superficial in our prayer and study, then our once keen vision of God will begin to blur.
If those without God's Spirit who heard God's Word died in the wilderness as punishment for disobeying God, how much greater will be our punishment for drifting away? To us, God says, "Pay attention!" Our chance for salvation is now! If we are not successful, then our hope is lost! Paul advises us to see the scope of what God is doing in our lives. We must constantly remind ourselves of His purpose for our calling. We must pray and study with that purpose at the forefront of our minds.