What does Hebrews 12:25 mean?

In the verses just prior to this statement, the writer of Hebrews compared the old covenant to the new covenant, in terms of how each is presented to mankind. The old covenant was meant to teach mankind something about the holiness of God, and our inability to live up to His perfection. So, the old covenant was given with fire, and smoke, and noise, and warnings at Mount Sinai (Exodus 19:9–20). In contrast, the new covenant in Christ removes the barrier created by our sin (Hebrews 9:24), and allows us to approach God (Hebrews 4:16) thanks to Christ's work as our mediator (Hebrews 12:24).

Here, the writer returns to a warning similar to the one given very early in this letter. In Hebrews 2:2–4, he points out that God delivered judgment on those who ignored His warnings, so greater consequences can be expected for those who ignore the superior new covenant. And, in Hebrews 10:26–31, this is given even more graphic explanation, showing that those who defy God, in the context of the new covenant, are exposing themselves to even greater consequences than those who know nothing at all.

The point made seems to be the same: our choices have consequences. This letter is written to Jewish Christians struggling under a certain level of persecution. The temptation they face is to go back to a more socially-acceptable religion. And yet, all of the evidence presented in the book of Hebrews is meant to show how foolish that would be. Christ, and His gospel, represent the one and only Truth; we can't expect anything but disaster if we deviate from that!