Galatians 4:22-31
For it is written that Abraham had two sons, one by the slave woman and the other by the free woman. 23 His son by the slave woman was born in the ordinary way; but his son by the free woman was born as the result of a promise.

24 These things may be taken figuratively, for the women represent two covenants. One covenant is from Mount Sinai and bears children who are to be slaves: This is Hagar. 25 Now Hagar stands for Mount Sinai in Arabia and corresponds to the present city of Jerusalem, because she is in slavery with her children. 26 But the Jerusalem that is above is free, and she is our mother. 27 For it is written:

"Be glad, O barren woman,
who bears no children;
break forth and cry aloud,
you who have no labor pains;
because more are the children of the desolate woman
than of her who has a husband."

28 Now you, brothers, like Isaac, are children of promise. 29 At that time the son born in the ordinary way persecuted the son born by the power of the Spirit. It is the same now. 30 But what does the Scripture say? "Get rid of the slave woman and her son, for the slave woman's son will never share in the inheritance with the free woman's son." 31 Therefore, brothers, we are not children of the slave woman, but of the free woman.

"You have come to Mount Zion, to the Heavenly Jerusalem, the City of the Living God. You have come to thousands upon thousands of angels in joyful assembly, to the Church of the Firstborn, whose names are written in Heaven. You have come to God, the judge of all men, to the spirits of righteous men made perfect, to Jesus the mediator of the New Covenant, and to the sprinkled blood that speaks a better word ..." ~Hebrews 12:22-25 NIV
Let's break this down and take a closer look at what this means. I see it describing the vast Christian Community at large, the assembly of all of Christ's People everywhere.
"You ..." - To whom is the author of Hebrews writing? It was to the same group of believers whom the author expected to personally visit along with brother Timothy per Hebrews 13:22-23 ("I have written to you in few words. Know that our brother Timothy has been set free, with whom I shall see you if he comes shortly."
"... have come ... " - notice how, at the time of the recording of this verse of Scripture in Hebrews, coming to this place was already a past tense experience for these believers, a done deal! Those first readers of the book of Hebrews were standing at the base of Mount Zion and hearing the call from above, to "come up here," just as the ancient Hebrews had been invited to do an Mount Sinai. Paul expresses his response to the upward call in Philippians 3:12-14.
"... to Mount Zion, to the Heavenly Jerusalem, the City of the Living God." - Can you see how by listing the names this way, it tells us that these are synonymous terms, this is one and the same place with a variety of descriptive names. We do the same thing today, we say, "Shy-town, Chicago, the windy city." Or we say, "The Big Apple, New York, the city that never sleeps."
"... You have come..." - He emphasises again that their coming to this place was already a past tense experience!
"... to thousands upon thousands..." - Now, that can mean "1000's plus 1,000's," at minumum 4,000. Or it could mean "1,000's times 1,000's" which
would mean a minimum of 4,000,000. No matter what, in God's City we have a vast population.
"... of angels ..." - Truly, this is the real City of Angels, (no wise confused with Los Angeles, CA). This City has a large angelic population, heavenly messengers, the servants of those who would inherit Salvation (Hebrews 1:14 "Are not all angels ministering spirits sent to serve those who will inherit salvation?").
"... in joyful assembly, ..." - This is a party! A blow-out! A festival, a jamboree, a state fair! "Joyful," that means there's laughing, cutting up, joking, playing around, congratulating, celebrating, singing, maybe dancing, food, leaving the cares behind and having a good old time!
"You have come ... to The Church ..." - Now we're talking! This special place we're describing is Church! Not just a church, but The Church, The Ecclesia (Greek), the vast community of Christians everywhere, wherever faith in Christ & love for one another is found. This passage of Scripture here in Hebrews 12:22-25 describes what it means to go to church!
More later ...

REWARDS Delivered at the Fall of Babylon (AD70 Jerusalem)