For instance, when an ekklesia wasn't assembled, no one wrote that an ekklesia might not be an assembly because the people who did assemble were not assembled seven days a week and twenty-four hours a day. An ekklesia must assemble because it was an assembly. If it didn't assemble, it wouldn't be an assembly. When it assembled, it was an assembly. Men who didn't assemble were not an assembly.
If someone wrote he ekklesia, or "the assembly," no one would be confused about the singular noun. They wouldn't assume the use of the singular, "the assembly," meant there was just one, or that there was one invisible assembly, and then there were the visible assemblies, which were merely visible manifestations of the invisible one, except as Plato might have talked about any visible thing. However, Plato would have understood "the assembly" as the idea of assembly that would find its reality in an actual assembly (if he ever even did refer to ekklesia in a "Platonic" sense). The idea would, however, be an idea of an actual assembly that found its reality in a visible assembly of people. The idea and the real thing would not have been different in nature.
Plato uses ekklesia (ecclesia) in his Dialogues. When not referring to a particular assembly, and yet still an assembly, he uses "the assembly" (he ecclesia). We read this very usage in his Dialogues (click on link):
And I say that if a rhetorician and a physician were to go to any city, and had to argue in the Ecclesia or any other assembly as to which of them should be elected state-physician...
Notice that Plato uses the generic use of the singular noun, "the assembly" (the ecclesia). It is still an assembly. Notice that he says, "or any other assembly." "The Ecclesia" distinguished one kind of assembly from another. But both are still an assembly. This is actually quite simple. No one would think of turning even Plato's usage into something universal, invisible, and mystical.
When Jesus used ekklesia in Matthew 16:18, it's first recorded biblical usage, people hearing it in that day would have still understood it as an assembly. However, Jesus differentiated His assembly from other assemblies by saying, "my assembly." Despite saying "my" and using ekklesia in the singular number, an ekklesia was still an assembly. Jesus' further usage of ekklesia would support that point.
Hebrews 12:23 uses the Greek word ekklesia: "To the general assembly and church (ekklesia) of the firstborn, which are written in heaven, and to God the Judge of all, and to the spirits of just men made perfect." People would have no contextual reason to think of this as anything but an assembly of people in a particular location. There is the assembly of God at Corinth. This is the assembly in heaven. "Firstborn" are those who inherit heaven, saved people. As saved people, their names are recorded at one point in time in heaven with the results of that recording permanent (perfect passive participle). That sounds like the book of life.
Does the church in the New Testament include Old Testament saints? No. Are Old Testament saints included in the firstborn? Are Old Testament saints in the book of life? If the church is New Testament, then the assembly of all of the firstborn of both the Old and New Testaments could not be New Testament. "Firstborn" itself is an Old Testament concept, very familiar to the Jewish audience of Hebrews. It is not intended to communicate something universal and mystical. An incentive for the audience of the book of Hebrews, in part unsaved Jews, was to be saved, to be converted, so to be in that assembly with all of the firstborn, both Old and New Testaments. Will Old Testament saints also inherit His eternal kingdom? Yes. Are the Old Testament saints in "the church"? If not, because Israel and the church are not the same, then how could "the firstborn" be the church of the New Testament? That ekklesia is merely an assembly in heaven, again as an incentive to unsaved Jews to get saved, so they can join the rest of the firstborn in their inheritance with the rest of the congregants in heaven, a congregation of Old and New Testament saints.
To be added to a New Testament church, someone must be baptized (Acts 2:41). Salvation is not the only requirement. Were Old Testament saints baptized? No. They were not baptized. Baptism is not a prerequisite for being firstborn. We inherit eternal life, the kingdom of God, by faith alone. But Old Testament saints will be in the assembly in heaven.
All saints can assemble in heaven. However, just because they are assembled doesn't mean that they are an ekklesia in the same sense as most of the usages of ekklesia, which are Jesus' assemblies on earth. His assembly on earth is an organized congregation of baptized believers, having New Testament officers, practicing New Testament ordinances, and is actively engaged in carrying out the Great Commission. The assembly in heaven isn't functioning like that. However, what it has in common with that earthly assembly is that it is local and it is an assembly. Not every use of ekklesia is an assembly in the sense of the church at Corinth or the churches of Galatia (cf. Acts 19:32, 39, 41), but every usage is still an assembly, because ekklesia means "assembly."
"Church" ("assembly") is a local and visible concept, which is why that word. Some ask, is there anything more than local in the New Testament? Yes. You have the kingdom of God and the family of God -- those are two universal concepts. They are both soteriological. Church, which is ecclesiological, is not universal. It is the opposite of universal. It is mutually exclusive from universal. If you are looking for something more than local, you've got it. You have kingdom of God and family of God. If you want universal, you don't need "church." "Church" is "assembly."
When you refer to an assembly, you are referring to something that a certain people do. They assemble together. They are an assembly because they assemble together. If they do not assemble together, then are not an assembly. They are not assembly until they assemble. If they are an assembly, they will continue to assemble. An assembly in heaven has never assembled. It will not be an assembly until people assemble in heaven. It is an assembly in prospect, but it should not be confused with what is the assembly on earth. That one is assembling now and it is made up of more than just saved people, but saved, baptized people, who gather to fulfill the purpose of the earthly assembly. They have to be in a particular locale, because an assembly only occurs in a particular locale.