How many of us would be in this category, ‘dull of hearing’? There is much to learn and the teaching of the word of God gives so much light concerning Christ, and we can see him and the deep things of God, but perhaps you are not ready. You cannot take anything complex; you do not desire it. Like any good teacher, the writer to the Hebrews is unwilling to teach material that is too advanced for his hearers. It will be a waste of effort and may even backfire. It will not be understood properly, it may make them heady, and it may raise all manner of difficulties and questions in their minds which reduce their confidence in their teachers.
The comparison is between Aaron and his sons, between their priesthood and the priesthood of Christ. Aaron's sons were only high priest for a time; Christ is the eternal high priest. Aaron and his sons were priests, but they were not kings; only Christ combined the office of high priest and king. Aaron and his sons were sinful, and needed to offer for themselves for their sins; Christ did not need to offer for his sins, but only his obedience which he offered for all his people. The Aaronic high priests offered only beasts and birds, but Christ offered himself, his own spotless perfect blood were. The priests of Aaron offered frequently, but Christ offered once.
Today, you get tremendous errors coming in and affecting vast numbers of people in churches, because the people are not learning the basics. For example, among the first things of the oracles of God, would be truth. The fact that the apostles performed signs and wonders as, what Augustine called, betokenings, or authentications, that they really had seen the Lord, and that many of them were bearers of inspired Scripture. So they needed betokenings, authenticating signs. And people forgot that basic truth. And so around the year 1900 Pentecostalism comes in. Gently at first, earnest people, thinking we ought to have the gifts today, because they didn't know the basics, and so you get a whole sector of evangelicalism goes off at a tangent with one cause. And there are many instances of this where basic truths of Scripture are not appreciated. This was evidently so with the Hebrews. They didn't clearly enough understand justification by faith alone, and so they were vulnerable to this idea that you need to add to Christ the Jewish ceremonies. The regarded the high priests of old as very special, and thought the angels played a vital role also, and there are all sorts of fables about that. So they are chastised for this: for not knowing or retaining the first principles.