Check out my children's book here: https://www.amazon.com/dp/B08FP9NZNK The contrast here is between Mt. Sinai and Mt. Zion. Mt. Zion refers to Zion in the kingdom age (cf. Psa. 2:6; 48:1-2, 8; 102:12-22; 132:11-18; 146:10; Isa. 2:1-5). The heavenly Jerusalem is in existence now and will be related to the millennial earth and on the eternal earth (Gal. 4:24-31; Heb. 11:10, 16). After referring to the heavenly Jerusalem in verse 22, the writer describes its inhabitants in verses 22b to 23. Five categories are given: (1) Myriads of angels in festal assembly, (2) the church of the firstborn (the church of Christ in the N.T. age), (3) God, (4) the spirits of righteous people made perfect (O.T. saints), and (5) Jesus.