This is from a teaching website and a standard interpretation.
A link to the site is at the end.
What Does The Church of The Firstborn Mean?
Hebrews 12:22-24 states: But you have come to Mount Zion and to the city of the living God, the heavenly Jerusalem, to an innumerable company of angels, to the general assembly and church of the firstborn who are registered in heaven, to God the Judge of all, to the spirits of just men made perfect, to Jesus the Mediator of the new covenant, and to the blood of sprinkling that speaks better things than that of Abel.
First, in the original language, the term “firstborn” is in the plural. It should be translated as “church of the firstborn ones.”
Second, we must recognize that Jesus is The “Firstborn.” He was given the preeminence (superiority) by God the Father over the new creation (Colossians 1:15-20). He was also preeminent because He arose from the dead and was the first person to die no more (Revelation 1:5).
Third, sinners who trust and obey the gospel of Christ (John 8:24; Acts 17:30; Romans 10:9,10; Acts 2:38) will undergo water baptism to identify with Jesus (Ephesians 1:3,12; 2:13; Acts 22:16 – His precious blood which washes away our sins), the Firstborn One, to also become firstborn ones because they have been raised from water “to walk in newness of life” (Romans 6:3,4). They have experienced a death to the old life of sin, a burial of that old man of sin, and a spiritual resurrection of a new man by God’s grace through faith via water immersion. They also look forward if they are faithful (if they should die first) to a physical resurrection from the dead at the end of time (1 Corinthians 15) since Jesus was the first one to rise from the dead to die no more.
Discussion and Analysis
First, the fact that firstborn is in plural form is not Biblical interpretation, but Grade school grammar. The subject is ‘church’ which should require no explanation, as having many members. This requires the plural form in the object of the preposition, ‘firstborn.’
Second, Jesus is identified as the Firstborn, hence the church of the firstborn implies: His church, His body. This then supports the idea of ‘The Church of The Firstborn One’. Translating ‘church of the firstborn ones’ suggests to the reader that only firstborn children are allowed membership, and misrepresents the significance of the word.
Third, and most elaborate is the explanation that sinners who are baptised in repentance become ‘firstborn ones’ by faith. No where in the New Testament are we ever instructed that we have become firstborn by grace through faith. To understand the significance of the firstborn, the seeker of Truth must look to the Law and the Prophets.
Isaiah 8:20. To the law and to the testimony! If they do not speak according to this word, it is because there is no light in them.
[Here is an analysis based on Exodus 13 and associated scriptures.]
Memorial of the Firstborn
Exodus 13:9. And it shall be for a sign unto you upon your hand and for a memorial before your eyes that the Lord’s law may be in your mouth; for with a strong hand the Lord has brought you out of Egypt.
After destroying all the firstborn males of Egypt and sparing Israel, both of man and beast, the Lord claims all the firstborn males who open the womb as His own possession. To eat bread without leaven for seven days is to acknowledge what God sanctifies unto Himself as an ordinance forever. These commandments are for a memorial to the strong hand of the Lord in our life, to direct our actions in accordance with His ways, not our own ability.
Exodus 13:1-8, (abbreviated). The Lord spoke to Moses; sanctify unto me all the firstborn…remember this day you came out of Egypt, for the Lord brought you out with a strong hand, therefore you shall not eat with leaven….for seven days…and you shall tell your son in that day “This is done because of that which the Lord did unto me when he brought me out of Egypt.
Exodus 13:14, 15. And it shall be when your son asks you in time to come, what is this? You shall say unto him, with a strong hand the Lord brought us out of Egypt, the house of slavery…..therefore I sacrifice to the Lord every male that opens the womb but all the firstborn of my children I redeem.
The Lord displays His strong hand in judgment against the strength of man as a display of sovereignty.
1 Corinthians 1:25. “For the weakness of God is stronger than men.”
This perception of man invested in the first born male is clearly understood in the words of Jacob in his final blessing upon his sons.
Genesis 49:3. Reuben you, my firstborn, my might and the beginning of my strength, the excellency of dignity and the excellency of power.
“The beginning of my strength” is the expression of a man’s virility.
Psalm 105:36. He smote all the firstborn in their land, the chief of all their strength.
On the other hand, God displays His might and the beginning of His strength in creation:
Isaiah 40:26. Lift up your eyes on high and behold who has created these things, who brings out their host by number; He calls them all by name by the greatness of His might, for that He is strong in power; not one fails.
Moreover, this perception of the firstborn male is an expression of the pride of man. In this context, the Lord invests Israel before the witness of Pharaoh (the false man-god) with a severe warning:
Exodus 4:22&23. And you shall say unto Pharaoh, thus saith the Lord, Israel is my son, my firstborn, and I say unto you; let my son go that he may serve me, and if you refuse to let him go, behold, I will slay your son, your firstborn.
The priesthood became exclusive to the tribe of Levi after the golden calf incident, and as a result, they took the place of the firstborn redeemed of the Lord.
Numbers 3:12 & 13. Behold, I have taken the Levites from among the children of Israel instead of all the firstborn that open the womb from among the children of Israel, therefore the Levites shall be mine because all the firstborn are mine; for on the day that I smote all the firstborn in the land of Egypt I hallowed unto me all the firstborn in Israel, both man and beast, mine shall they be: I am the Lord.
The census established the number of Levite males qualified to serve with Aaron in the priesthood, the sons of Gershon, Kohath, and Merari. The census of the firstborn males totaled 273 souls above that of the qualified Levite males, and these were redeemed by a five shekel per man redemption price.
In modern day Israel, the government mints a five shekel coin that can be purchased for a family to commemorate a son as the firstborn, by natural childbirth. Family restrictions in some countries, birth control, abortion and cesarean birth all restrict the number of firstborn males who open the womb, and thus the church of the firstborn.
Take heed to understand that the church is not exclusive to firstborn males of natural childbirth, but the number establishes a limit to the fullness.
God has taken for Himself the firstborn reflecting the fact that the Son of God is the firstborn of all creation, as well the firstborn from the dead.
Luke 21:24. For Jerusalem will be trampled upon by the gentiles until the time of the gentiles be fulfilled.
Romans 11:25. For I do not desire, brethren, that you should be ignorant of this mystery, lest you should be wise in your own opinion, that blindness in part has happened to Israel until the fullness of the Gentiles has come in.
Thus these prophetic scriptures apply to the gentiles, and this age has an established end.