The author of Hebrews wants to tell his audience more about Jesus’ role as priest in the order of Melchizedek, but he can’t. And the problem is that the Christians he is writing to are slow of hearing and infants in their knowledge of deep things.
He begins by declaring that he knows why they cannot understand. Because of their choice, they have not progressed in their understanding of the deep things of the Gospel. He calls them to study and grow.
He calls them to move beyond the basics and to deepen their understanding of those foundational truths. And he reminds them that God is the one who empowers their growth in knowledge and grace.
He finishes with a warning for those who “fall away” or turn their back on Jesus. This is a dire warning meant to move the Hebrew Christians to search their heart and find true faith there.