The authorship of the letter to the Hebrews is unclear. Many people think Paul probably wrote it, but it’s all speculation, really. In any case, the writer sometimes lets his frustration show.
As the letter is intended primarily for the Hebrews, it has a lot to say about how Jesus fulfils the meaning of the Hebrew scriptures (the Old Testament). In Chapter 5, the author explains how Jesus is basically the ultimate High Priest (and since He’s finished the job, we don’t need priests any more). But when he finishes, the author turns his attention to the recipients:
“We have much to say about this,” he says, “but it is hard to make it clear to you because you no longer try to understand.”
Then, if that wasn’t scathing enough, he continues:
“In fact, though by this time you ought to be teachers, you need someone to teach you the elementary truths of God’s word all over again. You need milk, not solid food!”