11 of whom we have much to say, and hard to explain, since you have become dull of hearing.
At this verse and running through 5:19 the author digresses from the priesthood of Christ. At 6:20 he takes up the High Priesthood of Christ again. The section running from 5:19 to 6:20 is a warning, a caution about not growing into maturity. A Christian cannot mature without a proper view of the person and work of Christ.
11 of [about] whom [Melchizedek] we have much to say,
There is much more to say about Christ’s Melchizedekian priesthood. He has additional things to say “about this” subject. He will do this in Hebrews 7:1-10:18. However, in order to grasp the significance of this Priesthood the readers of Hebrews need a level of maturity to do so. This is addressed in this and the next chapter.
and hard to explain,
The Melchizedekian priesthood is “hard to explain” to people who are indifferent to it.
since [because] you have become
The Greek word “become” means to become something that it was not before. At one time the readers of Hebrews were eager to learn about Christ but now they have lost that motivation. They were not always slow to pursue spiritual things.
The words “have become” then indicate that the readers began the process of negative volition toward God’s Word in the past with the result that they are in a state of “dull of hearing” (perfect tense).
dull of hearing.
The word “dull” means hard to move or no push. The people indicated here were sluggish or slow to come to grips with the supremacy of Christ. Jewish Christians reading the book of Hebrews were intransigent toward New Testament truth. They were listless about the wonder of Christ (He 6:12).
“Hearing” relates to how they were learning. Hearing God’s Word is vital in importance (He 2:1-4; 3:7-19; 4:12-13). The idea is more than nodding off when the Bible is taught but failing to understand its principles and applying them to oneself. The issue was more than laziness but spiritual resistance toward God (Jer 6:10).
Reluctance to apply God’s Word leads to a state of immaturity.
Many Christians have a case of arrested spiritual development. Unresponsiveness to God’s Word is a serious malfunction in Christian living. It results in immaturity.
An important manifestation of immaturity is dullness toward God’s Word, a resistance to what God has to say. Spiritual lethargy will blunt spiritual progress. Borderline believers are vulnerable to spiritual coldness that will result in immaturity.
Our hearts will harden toward God by hearing His truth and not acting on it. This is subtle insulation of our hearts against His Word. It is difficult to teach these people anything because they are not open to what God has to say; they are spiritually shut down. These people were at one time positive to God’s viewpoint because they were “once enlightened” and “tasted of the heavenly gift” (He 6:4). They once carried positive volition toward God but now they are in reversion.