What does Hebrews 5:12 mean?

The writer of Hebrews wants to give a detailed explanation of how the High Priesthood of Jesus Christ is superior to any other. And, he will do just that, in chapters 7 through 10. For now, though, he pauses to point out that some of this deeper meaning will be lost on his intended audience. In the previous verse, this was blamed on being "dull," which was not a reference to intelligence. Rather, the term nōthroi infers someone who is slow-moving or lazy. The Jewish Christians the writer speaks to have become sluggish and apathetic in their spiritual growth.

In these verses, the writer explains their flaw through the analogy of a child's development. Infants have to survive on milk, because they are not mature enough to process solid food. Over time, however, the child's body grows and they need to move on to something other than just milk. In the same way, a Christian's spiritual growth has to begin with the simpler things (1 Peter 2:2). Over time, spiritual growth should lead a believer to understand ideas with greater substance (1 Corinthians 3:1).

As the writer says, the Christians to whom he is writing have more than enough time invested. They should be able to teach others by now, but instead they are still spiritual infants. Why? Because they have been slow-moving and lazy in their approach to the faith: "dull of hearing." Instead of teaching, they still need to be taught, as if they were only new converts to the faith!