Here begins the final paragraph in the final warning against apostasy. On the similarity of vocabulary look at verse 19 in this chapter and 2:3. In verse 19 we had the word "bear." Here it means "to reject, to refuse." Verses 19-21 picture Israel at Sinai, convicted of sin by the Law. But here we have Israel later, in the wilderness when they rejected God.
Verses 25-26 are a lesser to greater argument.
This refers to Sinai. It must have been a frightening experience. The quotation from Haggai is Messianic. And from the context we learn that this passage plainly has eschatological overtones. Nothing has shaken earth and heaven as did the first coming of the Messiah. And it will shake again when He comes again the second time.
The lesser to greater argument takes us from fear for the Law to joy in the Gospel.
"Therefore" applies immediately to verses 25-29 but, in a sense, goes back almost to the beginning of the book of Hebrews. "Are receiving" is plainly causal and note that it is present tense. We are already receiving. The "unshakeable kingdom" reminds us of Luther's words: "the kingdom ours remaineth."
This verse exhorts us to worship God "pleasingly." What a wonderful thought! Verse 28 makes a wonderful Thanksgiving text. We should be thankful and full of pleasing service because we are constantly receiving an immovable kingdom. We must do this in view of the imminence of the final catastrophe. "Let us serve God pleasingly with reverence and awe."
The last phrase impresses on our minds the fact that sanctification is not optional. The fruitless person must face a consuming God. Not translated here in Greek is a "for to be sure."
This section ends on a note of warning. But the motive for verses 18-24 is not fear but rather confidence. The point is that the believer has an utter fear of giving up this confidence. Look at Luke 21:29-33 and verses 34-36. Note that the writer calls him "our God." It implies faith and confidence. Because of his faith the believer will stand when all else is consumed.