In the words of John Piper:
“At the Last Supper Jesus took the cup and said, “This is my blood of the covenant which is poured out for many for the forgiveness of sins.” The “many” is the church, the new Israel, the elect. So what happened when Jesus died was God’s final, decisive, sovereign, invincible act to create a people for himself—not only by purchasing their forgiveness, but also by purchasing their faith and their obedience in fulfillment of the new covenant promises: “I will put my laws into their minds, and I will write them in their hearts.”
You and I exist as Christians because of the irresistible force of the new covenant promises. The new covenant creates the church and guarantees that the gates of hell will not prevail against it and nothing will separate it from the love of God. God has given the church to his Son as a bride by a sacred marriage oath called the new covenant. And nothing will destroy this union.
That’s the ultimate reason why it is fitting for local churches to be formed as covenant communities—assemblies that covenant to be the church for each other. Christ has created us by a covenant to be his people, not just individuals, but a people—a body, a bride—for himself. To fulfill that covenant calling demands that we form assemblies called churches—visible churches—which in some way make a covenant together to be the body of Christ for each other, for the world, and for the glory of God.”