When you have a single text for a sermon, you also need a fence. The fence is there to keep you from wandering too far away from your focus.
Erect a fence for the passage – last night my preaching text was Hebrews 13:20-21, the final benediction. I erected a fence around the book of Hebrews. That fence meant that I kept my study in Hebrews and my presentation in Hebrews.
Study inside the fence – So what did the writer mean by the reference to “Shepherd,” “the will of God,” and “pleasing”? While naturally my mind might jump to Psalm 23, John 10 and other passages all over the canon, I tried to stay within the fence. The best evidence of authorial intent would be found in Hebrews. By staying there I discovered the unity of 13:1-21 as a follow-on to 12:28, which shed light on “pleasing.” By staying there I discovered the unity of the final section with parallels to the end of chapter 10, which shed light on “the will of God.” Staying within the fence kept the focus for study.
Preach inside the fence – It is always tempting to present the sermon in the terms you prefer. I tried to preach in Hebrews terminology. Instead of talking about our “vertical spirituality” as loving God (as I would by default, very Johannine), I instead spoke of worshipping God – very Hebrews. References to a pilgrimage of faith, toward a heavenly city, not shrinking back, shame, the joy set before, Jesus’ being led up from the dead, and so on. All terminology appropriate for a sermon on Hebrews. I also tried to refer to the writer as the preacher to the Hebrews rather than the standard writer to the Hebrews.
You only need a low fence – I am not suggesting that you study or preach a book in complete isolation from the other inspired texts. I am suggesting you honor the author of the book in both your study and presentation. So to understand “Shepherd” I had to be aware of at least Isaiah 63:11 in the LXX, although the addition of “Great” is very much a Hebrews idea. And to see that God is pleased with the two-part sacrifice of vertical and horizontal spirituality naturally sets up a brief comment about the greatest commandment(s), John’s first epistle, etc. The fence does not preclude very helpful study in Old Testament quotes and allusions, nor the opportunity to point out the consistency of idea across New Testament books. The low fence is there to honor the author, thereby helping you study better, and present more faithfully.