Answer
The phrase church of the firstborn is found in Hebrews 12, a chapter that speaks to the differences between Mt. Sinai (where the law was delivered) and Mt. Zion, the dwelling place of God. Verses 18–21 recall the giving of the law on Mt. Sinai and the fact that no one was allowed to approach the mountain or even touch it. There was fire and smoke, and the people were terrified (see Exodus 19:10–19). Only Moses was allowed to go up the mountain to meet God.
This scene at Mt. Sinai is contrasted with the New Covenant, symbolized by Mt. Zion—not the earthly mountain but the heavenly counterpart. (Mt. Zion was the site of Jerusalem and came to stand for the place where God resides.) Generally speaking, the tone of the passage describing Mt. Zion is much less ominous and more joyful. Not only does God “reside” in the heavenly Jerusalem, but also there are “angels in joyful assembly” (Hebrews 12:22) and “Jesus the mediator of a New Covenant” (verse 24). With them all are “the spirits of the righteous made perfect” and “the church of the firstborn, whose names are written in heaven” (verse 23). So, under the New Covenant, there are many who are able to approach God, unlike what happened on Mt. Sinai.
“The spirits of the righteous made perfect” refers to all believers but may, at least temporarily, refer to those who have already died in the Lord—they are now made perfect. The “church of the firstborn” is comprised of all those whose names are written in heaven—believers of all ages, dead or alive. The Greek word for “church” simply means “assembly,” and that’s how the NLT and CSB translate it.
But why is it called the church of the “firstborn?” Does firstborn describe the members of the church in some way? Or does firstborn refer to someone else?
In the New Testament, Christ is described as “the firstborn over all creation” (Colossians 1:15). This refers to His position as rightful heir over all creation. This seems to be similar to the way the word is used in Hebrews 1:6.
Colossians 1:18 and Revelation 1:5 speak of Jesus as the “firstborn from the dead.” This refers to the fact that He is the first to be resurrected and given a resurrection body; it also implies that many more resurrections will follow. Romans 8:29 says that Christ is the firstborn of many brothers and sisters. So, the “church of the firstborn” is simply another way of saying the “gathering of people in Jesus Christ.” The term firstborn emphasizes that He is preeminent over all creation and the first of many to be resurrected and given a glorified body.
The passage ends with a warning. If those who rejected the Old Covenant were punished severely, how much more severe will be the punishment for those who reject the superior covenant?