Article by Dr. Derek Thomas (original source here)

In the language of the Westminster Confession of Faith, the church comprises the “whole number of the elect, that have been, are, or shall be, gathered into one, under Christ the Head thereof” (25.1). This is otherwise known as the invisible church. In another sense, the church is the body of the faithful (1 Cor. 12:27; Eph. 2:21–22; Rev. 21:2, 9), consisting of those throughout the world who outwardly profess faith, together with their children (WCF 25.2). This is otherwise known as the visible church.

The Greek word that is translated as “church” in the Bible is ekklēsia. Conscious as we should be of the etymological fallacy (the idea that a word means what its composite root means), in this case it would seem to have merit. Thus, ekklēsia translates the Hebrew word qahal, the noun form meaning “assembly” or “congregation” and the verb essentially signifying “to call.”

Often in the Greek translation of the Old Testament, the word qahal is translated synagōgē. Common to both Hebrew and Greek words is the idea of assembling together before the Lord. Thus, the Bible translation of Paul’s day (the Septuagint) rendered Deuteronomy 4:10 (“assemble the people before me”) using the word ekklēsia — the gathering together of the Lord’s people as a covenant community before their covenant God.

Taking this etymological clue, we can expand what the word church in the New Testament means along three lines of thought:

First, the preposition ek (or ex) in ekklēsia suggests a particular dimension to the meaning of the word: the church is an assembly of people called out of the world. The church comprises those who are “called to be saints [holy ones]” or, possibly, “the holy called ones” (Rom. 1:7; 1 Cor. 1:2), just as their Old Testament counterparts were called “a holy nation” (Ex. 19:6). As the Niceno-Constantinopolitan Creed (381 AD ) affirmed, the church is “one, holy, catholic and apostolic.” We are “set apart as holy” (2 Tim. 2:21); we are chosen to be holy (Eph. 1:4); we are “God’s chosen ones, holy and beloved” (Col. 3:12), “a holy priesthood” (1 Peter 2:5), “a holy nation” (1 Peter 2:9), and “a holy temple” (1 Cor. 3:17). Brutally honest as we must be about the unholiness of the church, “the church is so holy that every one of its members is a saint” (Philip Graham Ryken).

Second, the church is an assembly called together into a homogenous, integrated unity. Several perspectives reinforce this in the New Testament. The church comprises the “family of God.” Each member of the church has become an “adopted son” (huiothesia; Rom. 8:15; 9:4; Gal. 4:5; Eph. 1:5). Now we are “members of the household of God” (Eph. 2:19), in which Jesus Christ is our elder brother. Jesus is not ashamed to call us brothers (Heb. 2:11). We come to God in prayer, saying, “Our Father” (Matt. 6:9). To those whose experience of family is dysfunctional in this world, the experience of belonging to a community of brothers and sisters is redemptive and restorative, particularly when they experience the loving concern (fellowship [koinōnia]) of “those who are of the household of faith” (Gal. 6:10).

Third, the church is comprised of those who are called into fellowship with the Lord. The church of God lives in God’s presence. Paul, addressing the issue of the need for orderliness and interpretation in the use of the Apostolic gifts of prophecy and tongues, adds the remark that when these gifts are correctly used, an unbeliever will be forced to declare that “God is really among you” (1 Cor. 14:29). From the very beginning, the community of the Lord’s people was called together in order to worship the Lord (Ex. 3:12). The primary relationship is vertical, not horizontal.

This brings us to the nature of the church as both here and there, on earth as well as in heaven: “But you have come to Mount Zion and to the city of the living God, the heavenly Jerusalem, and to innumerable angels in festal gathering, and to the assembly of the firstborn who are enrolled in heaven, and to God, the judge of all, and to the spirits of the righteous made perfect, and to Jesus, the mediator of a new covenant, and to the sprinkled blood that speaks a better word than the blood of Abel” (Heb. 12:22–24).

The church, then, consists of those whom the Lord has called out of the world into union and fellowship with Christ and into communion with each other. And, as John Calvin (citing the church father Cyprian) says, “To those to whom he is a Father, the Church must also be a mother.” (Institutes 4.1.1). I wonder if you would agree with him.