Hebrews 12:22 begins with a clear and stark contrast with Hebrews 12:18–21, where the author relates how impressive and terrifying the appearance of God was on Mount Sinai for Moses and the people. At Sinai it was no trespassing
for the Israelites to come close to God, but now—thanks to Christ—believers can come close to God in heaven!
The witnesses of Hebrews 12:1 return in Hebrews 12:23: the spirits of the righteous made perfect.
Hebrews 12:22–24 stands in the whole of the letter with the clear call to be steadfast in the faith and follow Jesus (see the introduction of Hebrews 12:1–3).
And so, Hebrews 12 concludes (in Hebrews 12:25–28) with an emphatic call to not reject the Word of God from heaven, and to receive the unshakable kingdom specifically with reverence and awe.
God is and remains a consuming fire (Hebrews 12:29) for those who (continue to) reject him—a clear reference to the appearance of God on Mount Sinai, and therefore also a connection with the previous section. This fire indicates the same seriousness as in Hebrews 10:26–31, where it deals with the terrible results for those who spurn the Son of God and who outrages the Spirit of grace—an important theme in this letter!
1 Therefore, since we are surrounded by so great a cloud of witnesses, let us also lay aside every weight, and sin which clings so closely, and let us run with endurance the race that is set before us,