In Hebrews 12:18, Mount Sinai stands as a representative for worship under the Law, and thus the location of this worship is first the tabernacle and later the temple. In contrast, worship for a Christian takes place on “Mount Zion, even the city of the living God, the heavenly Jerusalem” (12:22). These synonymous terms refer to the heavenly city, the place where God himself dwells, which is made clear by the groups of participants there: joyful angels, “the assembly of the firstborn,” God, and “the spirits of the righteous made perfect” (vv 22–23). The term ekklēsia, translated “assembly” in verse 22 likely refers to the common sense of New Testament “church” and thus designates post-Pentecost believers, while the latter phrase describes Old Testament saints now “made perfect” by the atonement of Christ’s blood.So the company of worshipers in this heavenly scene includes angels, New Testament Christians, and Old Testament believers. Here in this heavenly city God actually dwells; the worshipers come to him rather than him coming down to them as in the Sinai experience and his presence in the tabernacle and temple.
—By the Waters of Babylon, p. 125.