Question: What does "church" mean?
Answer: The word church is from the Greek word ekklesia. Ekklesia is not a religious word as originally used among the Greeks. Ekklesia is a compound word meaning "ek, out of, and klesis, a calling" (W.E. Vine, vol. 1 pg. 83). The word is used in its basic, secular sense in Acts 19: 32, 39 - an assembly and, then, a lawful assembly.
The Holy Spirit spiritually used the word church. The first time ekklesia (church) is used in the spiritual sense is in Matthew 16: 18, where Jesus promised to build His church (ekklesia). Applied spiritually, ekklesia suggests the called out of - that is, the saved who are called out of the word (John 17). "Church" (ekklesia) is used to denote all the saved without any particular geographic designation (Matt. 16: 18), and of the saved in a particular geographic area (1 Cor. 1: 2). "Church" used universally (all the saved) is always used in the singular. "Churches," plural, refers to a number of local churches, the saved in geographic areas. Hence, Paul wrote, "... The churches of Christ salute you" (Rom. 16: 16). "Church" is applied to the assembly of God's people (1 Cor. 14: 19). "Church" denotes the saved who have banded together and have the full development as far as organization is concerned (Acts 14: 23). The local church, when fully organized has elders (bishops) and deacons (Phil. 1: 1; 1 Tim. 3: 1-13). "Church" is also used of the banded together saved in the absence of the full organization(Acts 14: 23).
How "church is not used. Out of the 115 times "church" is used in the Greek New Testament, it never refers to the building in which the church meets. "Church" is also never used to denote denominations. While it is true that denominations, as such, did not exist in the First Century, "church" is not applied to unauthorized religious groups such as the Nicolaitanes, etc. (Rev. 2: 6, cf. vs. 1).