The church is the community of all true believers for all time. This definition understands the church to be made of all those who are truly saved. Paul says, "Christ loved the church and gave himself up for her" (Eph 2:25). Here the term "the church" is used to apply to all those whom Christ died to redeem, all those who are saved by the death of Christ. But that must include all true believers for all time, both believers in the New Testament age and believers in the Old Testament age as well.
The starting point in ecclesiology is the ontological connection between Christ and the church. The church exists as church only insofar as it is Christ’s body, in union with him, meaning also our union in him, both of which are a matter of his free and gracious choice. The church has no other ground of being than Jesus Christ. This means in a primary way that the church is not the church as institution, or a voluntary collection of free, religiously and ethically motivated individuals, or, with its episcopate, as an historically ordered hierarchy that determines what it is and what it does. It is Christ alone who determines the “that” and the “what” of the church, who loves the church and calls and forms it according to his own purpose. The church is what he is in that he is Lord of the church in whom and from whom alone it has life. As such, the church belongs to Christ, not to itself. The church is not self-referenced. In a primary sense, its being is iconic, not institutional, as it points away from itself to Christ.
Andrew Purves Reconstructing Pastoral Theology (pg. 97)
[T]he church in Scripture is composed of all the redeemed in every age who are saved by grace through personal faith in the sacrificial work of Jesus Christ, “the seed of the woman” (Gen. 3:15) and suffering Messiah (Isa. 53:5–10).
Robert L. Reymond A New Systematic Theology of the Christian Faith (pg. 805)