For even as the body is one and yet has many members, and all the members of the body, though they are many, are one body, so also is Christ.
~ 1 Cor. 12:12
The church refers to all the people who belong to the Lord, those who have been purchased by the blood of Christ. Various other images and expressions are also used to define or describe the church. The church is called the body of Christ, the family of God, the people of God, the elect, the bride of Christ, the company of the redeemed, the communion of saints, the new Israel, among others.
The New Testament word for church, from which we get the word ecclesiastical, means “those called out.” The church is viewed as an assembly or gathering of the elect, those whom God calls out of the world, away from sin and into a state of grace.
Because the church on earth is always what St. Augustine called “a mixed body,” it is necessary to distinguish between the visible church and the invisible church. In the visible church (consisting of those who make a profession of faith, are baptized, and enrolled in membership of the institutional church), Jesus indicated there would be tares growing along with the wheat. Though the church is “holy,” it always, in this age, has an unholy mixture within it. Not all of those who honor Christ with their lips honor Him with their heart as well. Since God alone can read the human heart, the true elect are visible to Him, but in some measure invisible to us. The invisible church is transparent but completely visible to God. It is the task of the elect to make the invisible church visible.
The church is one, holy, catholic, and apostolic. The church is one. Though fragmented by denominations, the elect are united by one Lord, one faith, and one baptism. The church is holy because it is sanctified by God and indwelt by the Holy Spirit. The church is catholic (the word catholic means “universal”) in that its membership extends across the earth including people from all nations. The church is apostolic in that the teaching of the apostles as contained in sacred Scripture is the foundation of the church and the authority by which the church is governed.
It is the duty and privilege of every Christian to be united to the church of Christ. It is our solemn responsibility not to neglect the gathering together of the saints in corporate worship, to be under the nurture and discipline of the church, and to be actively involved as witnesses in the mission of the church.
The church is not so much an organization as it is an organism. It is made up of living parts. It is called the body of Christ. Just as a human body is organized to function in unity by the coworking and codependence of many parts, so the church as a body displays unity and diversity. Though ruled by one “head”—Christ—the body has many members, each gifted and endowed by God to contribute to the work of the whole body.
~ R. C. Sproul, from “Essentials of the Christian Faith”