When a believer comes to faith, they join the universal church. But what creates a local church? If we (rightly) do not call a couple of Christians bumping into each other at the shops a church, what is it that makes a local church, a church?
Bobby Jamieson offers this answer in an article at 9 Marks:
Recall Paul’s words in 1 Corinthians 10:16–17: “The cup of blessing that we give thanks for, is it not a sharing in the blood of Christ? The bread that we break, is it not a sharing in the body of Christ?” Paul reminds the Corinthians that to eat the bread and drink the cup is to enjoy fellowship with Christ, to experience the benefits of his death.
From this “vertical” fellowship between Christ and believers, Paul then draws a “horizontal” conclusion in verse 17: “Because there is one bread, we who are many are one body, for all of us share that one bread.” Paul’s central claim in this verse is that we who are many are one body. And he twice grounds or supports this assertion by referring to our joint participation in the Lord’s Supper: “Because there is one bread . . . for all of us share that one bread.” The fact that Paul repeats his reason twice weighs against seeing the bread as merely representing or picturing the church’s unity. Instead, Paul roots the church’s unity in its celebration of the Lord’s Supper. There is one body because there is one bread.
Paul is saying that the Lord’s Supper actually makes many one. The Lord’s Supper gathers up the “we who are many” and makes us into one body. In other words, the Lord’s Supper constitutes a local church. Of course, Paul’s point is not about the mechanics of bread and eating, as if a larger church that needed more than one loaf to celebrate the Lord’s Supper was no longer one church but many. Instead, Paul uses “one bread” as shorthand for the church’s corporate, all-together celebration of the Lord’s Supper. Paul’s point is that, in the Lord’s Supper, because we all share in fellowship with Christ together, our unity in Christ creates the unified body of the church.
In other words, the Lord’s Supper is the renewing oath-sign of the new covenant. In the Lord’s Supper, we renew our commitment to Christ and each other. And it is this twofold commitment that makes a church a church.
He goes on to apply it thus:
The ordinances of baptism and the Lord’s Supper play crucial roles. In baptism, you publicly commit to Christ and his people. Baptism is where faith goes public. It’s how a new believer shows up on the world and the church’s radar as a believer. In other words, baptism marks off a believer from the world. In baptism, the church says to the world, “This one belongs to Jesus!”
In the Lord’s Supper, we renew our commitment to Christ and his people. But, distinct from baptism, the Lord’s Supper is something we all do together. The Lord’s Supper marks off an entire group of Christians as one body, drawing a line between them and the world around them. And by drawing a line between the church and the world, baptism and the Lord’s Supper draw a line around the church. The ordinances make it possible to point to something and say “church” rather than only pointing to many somethings and saying “Christians.”
Imagine one Christian goes to a new city, preaches the gospel, and a handful of people all come to Christ around the same time. This new Christian baptizes each of them. How and when will this handful of baptized Christians become a church? I’d suggest the most basic, most essential answer is: when they celebrate the Lord’s Supper together. Remember that celebrating the Lord’s Supper expresses our commitment to Christ and to each other. To receive Christ’s benefits in the Lord’s Supper is to receive Christ’s people as brothers and sisters. In the Lord’s Supper itself we make the commitment to each other that takes us across the line between “handful of Christians” and “local church.” In the Lord’s Supper itself we come together as one body. As Paul says, “Because there is one bread, we who are many are one body, for all of us share that one bread” (1 Cor. 10:17)…
…A gathering of believers are not a local church until they seal their union with each other through the Lord’s Supper. If a group of believers who meant to be a church never celebrated the Lord’s Supper together, not only would they be disobeying Jesus, but there’s a real sense in which they would not yet be a church. The Lord’s Supper consummates the commitment by which Christians become a church.
How does the Lord’s Supper make a local church? Together with baptism, the Lord’s Supper is how a gospel people form a gospel polity. The Lord’s Supper is how Christians come together, commit to each other, and cross the line from “many” to “one.” In the Lord’s Supper, our fellowship with Christ creates fellowship with each other. The Lord’s Supper makes many one.