Caspar Olevianus (1536-1587) was another German Reformed theologian who had (at least) a hand in the formulation of the Heidelberg Catechism (1563). The following quotation makes clear that Olevianus was of the same mind as his teacher, Calvin; his friend, Beza; and his co-worker in Heidelberg, Ursinus:

When God provides our eyes with the sight of an assemblage which is a member of the H. Catholic Church, the mark having been shown of true prophetic and apostolic doctrine (under which are embraced lawful administration of the sacraments and training in all godliness, Matt. 28:20), we ought to unite with that assemblage. For as He is Himself our Father, it is His pleasure that the Church be our mother, Isa. 54:1-2, Gal. 4:27-28, 31. In her we are both born and brought up right to the end of our lives. God is pleased by the Church’s ministry to quicken us by His Spirit, stamp remission of sins on our hearts and reshape us daily in the same unto His own image. On the other hand he who despises such an assemblage possessing the mark of a true Church, to wit truth of prophetic and apostolic doctrine—which happens when a man does not communicate in sound doctrine and in prayers and when he does not attach himself to the communion of saints through the visible witnesses of the Covenant, baptism and the sacred eucharist—cannot be sure of his own salvation. And he who persists in such contempt is not elect, Acts. 2:47.

After commenting on the Apostles’ Creed’s article on "the communion of saints," Olevianus explicitly affirms extra ecclesiam nulla salus:

138 Q. How do you understand the possession of the benefits of Christ in this life?

I understand it as follows: just as there is no salvation outside the Church, which is the body of Christ, so also all true and living members of the Church now possess full salvation, that is, forgiveness of sins.

German-Dutch theologian, Peter van Mastricht (1630-1706) writes,

Query, whether any Christian, if he can, is bound to associate himself with any particular, fixed true Church. The Schwenkfeldians, Libertines, Enthusiasts and other fanatics, with whom also act the Socinians, say No. The Reformed recognise that there may be a hidden Church, since you cannot join any Church [i.e., because persecution is so fierce, no visible, instituted church can function]. But where you can, they lay it down that you simply must.

The two quotes above from Olevianus and van Mastricht are taken by Heinrich Heppe, a nineteenth-century German theologian and church historian, to be representative of the orthodox Reformed tradition.

Richard Muller, the foremost figure in this field today, in his Dictionary of Latin and Greek Theological Terms: Drawn Principally from Protestant Scholastic Theology, states that this is the standard position of the successors of the Reformers:

Extra ecclesiam non sit salus: Outside of the church there may be no salvation; a maxim from Cyprian (Epistles, 73.21) often cited by the scholastics, who accept it as true with the provision that the church is identified as the communio sanctorum (q.v.), or communion of saints, and by its marks, Word and sacrament (see notae ecclesiae). The maxim is also frequently given as Extra ecclesiam nulla salus or Salus extra ecclesiam non est.

Nineteenth-century Dutch theologian, J. J. Van Oosterzee (1817-1882) writes,

Indeed, the "Extra Ecclesiam nulla salus" is here the holy truth; men must belong to the little flock, if they will upon sure grounds solace themselves with the promise of salvation. The community of the saints is saving, not because everyone [who is a member of the visible church] is saved, but because he may be assured of his salvation, who knows himself a living member of the corpus mysticum.

Staying with the continental Reformed tradition but moving to the United States, we have the comments of R. B. Kuiper (1886-1966) in his work on the church, The Glorious Body of Christ:

In the first place, Scripture teaches unmistakably that all who are saved should unite with the church. The view that membership in the visible church is requisite to salvation has no basis whatever in Scripture. When the Philippian jailer asked what he should do to be saved, Paul said only: "Believe on the Lord Jesus Christ, and thou shalt be saved, and thy house." The apostle did not command him to join the church. However, when he did believe he was at once baptized (Acts 16:31-33). As soon as the Ethiopian eunuch confessed Christ he likewise was baptized (Acts 8:36-38). So were all who were converted at Pentecost. Now according to Paul’s words, "By one Spirit are we all baptized into one body" (I Corinthians 12:13), baptism signifies reception into the church. It is clear that in the days of the apostles it was universal practice to receive believers into the visible church.

What could be more logical? He who believes in Christ is united with Christ. Faith binds him to Christ. He is a member of Christ’s body, the invisible church. But the visible church is but the outward manifestation of that body. Every member of the invisible church should as a matter of course be a member of the visible church.

Extremely significant in this connection is Acts 2:47—"And the Lord added to the church daily such as should be saved." Not only does the Lord Christ require of those who are saved that they unite with the church; He Himself joins them to the church. And the reference is unmistakably to the visible church. Does it follow that he who is outside the visible church is necessarily outside Christ? Certainly not. It is possible that a true believer because of some unusual circumstance may fail to unite with the church. Conceivably one may, for instance, believe in Christ and die before receiving baptism. But such instances are exceptional. The Scriptural rule is that, while membership in the church is not a prerequisite of salvation, it is a necessary consequence of salvation. Outside the visible church "there is no ordinary possibility of salvation" (Westminster Confession of Faith, Chapter XXV, Section II).  ... to be continued