The writer to the Hebrews speaks of Mechisedec in 5:6-10, then in 5:11 diverts from the subject to make statements regarding those to whom the epistle is written and continues on that vein throughout the rest of the chapter and throughout the whole of chapter 6.
The subject then returns to what was previously addressed 'For this Melchisedec . . . ' in 7:1.
The whole section, 5:11 to 6:20, appears to be a digression and the writer returns to the subject in 7:1.
The things the writer is about to say are hard to be uttered for the reasons stated in 5:11 to 6:20. Only after covering why they are hard to be uttered, does the writer then utter them.
- 5:10 Melchisedec
- 5:11 Many things to say/hard to be uttered
- 5:12 - 6:20 Why they are hard to say/Remedy to that condition
- 7:1 - 7:28 The Utterance
- 8:1 Summary
The utterance is the whole of chapter 7. After which, 8:1, the writer sums up what has so far been covered 'Of the things which we have spoken, this is the sum'.
The utterance of chapter 7 covers the priesthoods of Melchisedec, Levi and Christ. These are the 'things which were hard to be uttered' before the writer digressed to deal with the spiritual state of those to whom the epistle was written. Specifically, the subject is Melchisedec ('of whom' in 5:10 is singular) but the reason for that being introduced at all is to make comparison with Christ in order to prove Christ's predominance over both Melchisedec and Levi.
The writer does this by referring to the mysterious priest in Genesis and thus shows to the Jews that the Levitical priesthood has been superseded by the superior priesthood of Christ - who fulfils every detail of the mysteries which were, so briefly, demonstrated by that fleeting appearance to Abraham, of that singular priest.
The reasons why the things are difficult to utter are because of the spiritual sate of the potential hearers, and the warnings, regarding continuing in such a state, are among the most severe warnings ever delivered in scripture.
I believe this is a very important matter. It is pointless trying to convey spiritual truth (in this case, spiritual truth about the relevant place of the priesthoods of Melchisedec, Levi and Christ) to those who are not in a condition, spiritually, to be able to receive such truth.