This passage can only be understood against the backdrop of the two covenants God made with His people (Heb.12:18-29). The first covenant was made at Mount Sinai after He delivered His people Israel from Egypt who had held them in bondage for four hundred years. The second, known as the New Covenant, was made when Jesus, its Mediator, shed His blood to deliver His people from their sins. In the first portion of this passage (12:18-24), the writer reminds his Jewish readers who had professed faith in Jesus that they are not under the Old Covenant (12:18-21), but under the New Covenant (12:22-24).
In the second portion of the passage (12:25-29), he warns them not to reject or turn away from the Lord Jesus Who had spoken to them, and secured through His blood the benefits of the new covenant. He made the same type of argument he made earlier in the epistle (2:1-4). If those under the Old Covenant did not escape judgment who refused the Lord Who spoke, how much more shall we under the New Covenant not escape His judgment if we refuse to hear and obey Him. The writer urged these Jewish believers throughout the epistle not to return to the Old Covenant but to continue following Jesus the Mediator of the New. We too, must continue in the faith, depending on the grace of God by which we may serve Him acceptably with reverence and godly fear.