The Son of God accomplishes the threefold work of gathering, defending and preserving the church – so Reformed churches testify in their catechism – by His Spirit and Word.

By the Holy Spirit Christ executes all that He must accomplish from Pentecost to the last day for the salvation of the world. This work the Spirit does through the Word, specifically the Gospel which He receives from Christ in order to proclaim it to men.

This gathering, defending and preserving of the church is actualized in the way of faith.

By the preaching of the Gospel the Holy Spirit works faith in the hearts of those whom God in grace chooses to be His children. By such faith believers are implanted or ingrafted into Christ.¹ According to this manner Christ imparts Himself to His people and so is appropriated and received by them.²

Faith embraces Jesus Christ and all His merits and lays hold upon Him. The Holy Spirit, dwelling both in Christ and in believers, unites believers by faith more and more with Christ so that they, although He is in heaven and they are on earth, never­theless are bone of His bone and flesh of His flesh. By that one Spirit they live forever and are governed, even as the members of one body are by one soul.³

This happens to all who come to faith.

Believers — each and every one — are members of the Lord Jesus Christ and have communion with all His gifts and treasures. ⁴

According to the catechism, therefore, all true believers are implanted and ingrafted into Christ.

All are members of His body ... Together they constitute by the working of the Spirit and the Word the body of which Christ is Head. ⁵And accord­ing to the express declaration of Paul that body of Christ is the church!⁶

The Reformed churches testify to that same truth in their confession. Therein they declare that they believe and confess one only catholic or universal church, which is a holy congregation of all true Christ-believers expecting the whole of their salva­tion in Jesus Christ, being washed by His blood, sanctified and sealed by the Holy Spirit.

This church has existed since the beginning of time, for it is preserved and upheld by God against all the rage of the world. Indeed, at times this church appears to be very small and seems threatened by destruction. But in spite of all this God sus­tains it. In Ahab's time the LORD gathered and pre­served in and in spite of apostate Israel — that is to say, in the midst of the false church of those days — the seven thousand who did not bend the knee to Baal.

This one catholic church, moreover, is not estab­lished in and restricted to one place or bound to specific persons. Rather it is spread and scattered throughout the whole world. But always by the power of faith it is joined together and united with heart and will in one and the same Spirit. ⁷

We also find this confession that the church is the congregation of all true believers in the Canons of Dort. There it is affirmed that God's counsel, flow­ing from His eternal love for the elect, is being powerfully accomplished from the beginning of the world even to the present day despite all the raging of hell. Therefore all the elect are being gathered together into one in His time. Thus there always will be a "congregation of believers," the foundation of which is laid in Christ's blood. ⁸

So the Reformed churches testify concerning Christ's church — and this they confess before aught else — that the church is the communion of ALL true believers. Or to say the same thing in other words: that ALL true believers are members of Christ's body, that is, of His church.

dest commentator on the Heidelberg Catechism, Casper Olevianus, wrote in his explanation of I believe one holy catholic Church: "Warum nennst du sie eine allgemeine Kirche? Darum, dass, gleichwie nur ein Haupt der Kirche ist nämlich Christus, also auch alle Gläubigen von Adam an bis zum Ende der Welt, sind seine Glieder, sind ein Leib durch den heiligen Geist, sind alle durch ein Haupt erlöset, einem Haupte eingeleibt and werden an einem Haupte erhalten durch den Glauben an ihn." Fester Grund christlicher Lehre, ed. Karl Sudhoff, Frankfurt am Main, 1857; S. 160. And Ursinus wrote in his Corpus Doctrine Christianae (his "Schat­boek" or well-known commentary), ed. MDCLI, p. 302: "Definitio ecclesiae in quaestione talis continetur: Ecclesia est coetus hominum ab aeterno electus a Deo ad vitum aeternam."