You can see what our writer is doing as he begins to wrap up his lengthy letter to these persecuted Hebrew believers. He's unpacked all the theology of the New Covenant. He's encouraged them through the details of the maturing plan of God even in the middle of their difficulties. Father God can bring a "peaceable fruit of righteousness" out of their present trials (12:11). But there's one thing more he wants to place before their discouraged minds.
When the journey is hard it helps to remember the destination. In today's text our writer reminds his readers - and us - that we didn't just sign on to a doctrinal statement or join an organization when we professed loyalty to Christ as our Redeemer and Lord. There is a bigger picture not yet visible. And it's easy to forget we entered into something more glorious than is physically visible or tangible right now.
And our writer's way of making his point is to paint a study in contrasts. Particularly for these Hebrew believers, he wants to show the folly of turning back to the Old Covenant through Moses and the law by contrasting it with what they came into through Christ and the New Covenant empowered by the Spirit of God.