What does Hebrews 12:26 mean?

After comparing the ominous start of the old covenant (Exodus 19:9–20) to the welcoming nature of the new covenant (Hebrews 12:18–24), the writer has issued another warning about the danger of falling away from the truth. Verse 25 made a statement similar to the words of Hebrews 2:2–4 and 10:26–31. In short, there will always be consequences for deviating from the truth; the better we know the truth, the harsher we can expect those penalties to be.

The prior verses compared the earthly scene at Sinai to the heavenly scene awaiting all Christian believers. Here, the writer once again invokes the way God spoke when He gave the old covenant at Mount Sinai. Psalm 68:8 recalls this event as one where the ground shook at His words. The following statement, beginning with "Yet once more…" is a quotation from Haggai 2:6, where God offers a long-term prophecy of His eventual remaking of all of creation (Revelation 21:1). This continues the same theme as the entire letter: that God will replace something temporary with something eternal. Just as the old covenant was replaced by the new, the old creation will be replaced by the new heavens and the new earth.