There is a battle between “present” us and “future” us - and though we don’t plan to fall apart or fall away, we often don’t plan not to. This is the same concern of the author of Hebrews: ch. 3-4 are one long extended argument to be faithful and persevere in holding fast to our faith in the present for the future enjoyment of God’s rest. But what -is- God’s rest, and how do we enter? Once more, Hebrews grounds our faith in Jesus himself.