Who constitutes the church?
The Christian church is the community of all true believers under the Lordship of Christ.
Catechism for the Global Methodist Church, question 28
19Therefore go and make disciples, baptizing them in the name of the Father and of the Son and of the Holy Spirit. 20Teaching them to keep everything that I have commanded you; and behold, I am with you all the days until the end of the age.
2to the assembly of God sanctified in Christ Jesus, that is in Corinth, to those called as holy ones together with all who call on the name of our Lord Jesus Christ in every place of theirs and ours. 3Grace to you and peace from God our Father and the Lord Jesus Christ.
11Therefore remember that you, the Gentiles in the flesh, who are called uncircumcision by the so-called circumcision in the flesh made by hands, 12that you were at that time without Christ, alienated from the citizenship of Israel and foreign to the covenants of the promise, having no hope and without God in the world. 13But now in Christ Jesus you who once were far off have been brought near by the blood of Christ. 14For he himself is our peace, who made both one and broke down the middle wall of the fence, the hostility, in his flesh, 15having abolished the law of commandments in decrees, so that he might create the two in himself into one new human, making peace, 16and might reconcile both in one body to God through the cross, having killed the hostility in it. 17And having come, he proclaimed peace to you who were far off and peace to those who were near, 18that through him we both have access in one Spirit to the Father. 19Therefore you are no longer foreigners and sojourners, but you are fellow citizens with the saints and members of the household of God, 20having been built upon the foundation of the apostles and prophets, Christ Jesus himself being the cornerstone, 21in whom the whole building, being fitted together, grows into a holy temple in the Lord, 22in whom you also are being built together into a dwelling place of God in the Spirit.
9After this I saw, and behold, a great crowd that no one was able to number, from every nation and tribe and people and language, standing before the throne and before the Lamb, clothed in white robes, and palms in their hands. 10And they cry out with a loud voice, saying, Salvation belongs to our God who sits on the throne, and to the Lamb.
After confessing the church's four marks, the catechism asks who belongs to it. The answer is the first sentence of Article V of the Confession of Faith: "We believe the Christian Church is the community of all true believers under the Lordship of Christ." The article continues by describing the church as "the redemptive fellowship in which the Word of God is preached by men divinely called, and the sacraments are duly administered according to Christ's own appointment," existing "for the maintenance of worship, the edification of believers and the redemption of the world."
The phrase "all true believers" is broad on purpose. Paul addresses his letter "to the assembly of God sanctified in Christ Jesus, that is in Corinth," and in the same breath "all who call on the name of our Lord Jesus Christ in every place" (1 Corinthians 1:2). John sees its final form: "a great crowd that no one was able to number, from every nation and tribe and people and language, standing before the throne and before the Lamb" (Revelation 7:9), crying "Salvation belongs to our God who sits on the throne, and to the Lamb" (Revelation 7:10). Membership is defined by faith in Christ.
"Under the Lordship of Christ" gives the church its shape. Ephesians describes Gentiles who "were at that time without Christ" and "without God in the world," who "have been brought near by the blood of Christ" (Ephesians 2:12-13). The result is a household "built upon the foundation of the apostles and prophets, Christ Jesus himself being the cornerstone" (Ephesians 2:20), "in whom the whole building, being fitted together, grows into a holy temple in the Lord" (Ephesians 2:21). The church grows by the commission Christ gave: "go and make disciples, baptizing them in the name of the Father and of the Son and of the Holy Spirit," teaching "them to keep everything that I have commanded you" (Matthew 28:19-20).
A Methodist learns from this article to look for the church wherever true believers gather under Christ, and to hold membership in the church as a fellowship of grace rather than a club. The vows of membership in the church, to support it with prayers, presence, gifts, service, and witness, are how a believer takes a place in the community this article describes.