Peter Y. De Jong (1915-2005), like R. B. Kuiper, was a conservative theologian in the Christian Reformed Church in N. America. Below is De Jong’s lengthy exposition of Belgic Confession Article 28’s statement that "out of it [i.e., the church] there is no salvation:"

In God’s word emphasis is laid upon our spiritual fellowship with Christ, which comes to expression in sound doctrine and pure worship. This insistence, however, may not tempt us to champion the notion that the external and visible form of the church is of little account. We learn to know God’s church only in and through its historical manifestation. More than that, the Bible warns against trusting our subjective judgments while disregarding and even despising the work of the Holy Spirit in the church of all ages. Always the individual and social, the personal and communal aspects of our salvation in Christ are interwoven in New Testament teaching. They do not exist side-by-side, in isolation from each other. To be a Christian means to have fellowship with the living Christ and in the same moment with his people. To break this fellowship lightly, on the basis of personal prejudices and insights, is to imperil our salvation. How else could we hear the word of the living God, except through the preachers whom he has sent? And how could such preachers receive their commission, except by the church which believes and lives by the word of God? Aptly does J. S. Whale comment, "Certain it is that for St. Paul, and for New Testament Christianity, to be a Christian is to be a member of a living organism whose life derives from Christ. There is no other way of being a Christian. In this sense, Christian experience is always ecclesiastical experience. The gospel of pardon reaches you and me through the mediation of the Christian society, the living body of believers in whose midst the redeeming gospel of Christ goes out across the centuries and the continents."

... Wherever the word is purely preached, there is the church. Constrained by the Spirit who indoctrinates us into the truth as it is in Christ, those who are saved live in fellowship with each other. Apart from Christ there is no salvation. And He is pleased to communicate His grace in connection with the means which He has instituted and preserved in this world. To separate oneself from the assembly where the rich Christ is proclaimed in obedience to the Scriptures is a heinous sin involving most serious consequences. "Hence it follows," so Calvin warned at this point, "that a departure from the Church is a renunciation of God and Christ. And such a criminal dissension is so much the more to be avoided, because, while we endeavour, so far as lies in our power, to destroy the truth of God, we deserve to be crushed with the most powerful thunders of his wrath. Nor is it possible to imagine a more atrocious crime, than that sacrilegious perfidy, which violates the conjugal relation that the only begotten Son of God has condescended to form with us" [Institutes 4.1.10].

Turning to the Presbyterian tradition, Scottish theologian David Dickson (1583-1663), the author of the first commentary on the Westminster Confession, included the following in his remarks on chapter 25, "Of the Church:"

Question 4. Is there any ordinary possibility of salvation out of the visible church?

No; Acts 2:47.

Well then, do not the Enthusiasts, Quakers, and Libertines err, who affirm, That any man may be a true Christian, and be saved, though he live within no visible church?

Yes.

By what reasons are they confuted?

1st, Because the Lord Jehovah in his visible church (ordinarily) commands the blessing, even life for evermore, Ps. 133:3. 2nd, Because the visible church is the mother of all believers, Gal. 4:26. By Jerusalem which is above, I understand the true Christian church which seeketh its salvation, not by the first covenant of the law, namely, by the works of the law, but by the second of the gospel, namely, by the merits of Christ embraced by a true faith; which hath its original from heaven, by the powerful calling of the Holy Ghost. 3rd, Because they that are without the visible church are without Christ, Eph. 2:12. 4th, Why are men and women joined to the visible church, but that they may be saved? Acts 2:47. 5th, Because they that are without the visible church are destitute of the ordinary means of life and salvation, Ps. 147:19, 20.

Nineteenth-century Scottish Presbyterian, Hugh Martin (1822-1885), in his fine commentary on Jonah, declares, "The Gentiles, as a whole, as nations, were obviously given over in the meantime to the reign of spiritual death, cast out beyond the pale of that visible church, within which alone salvation is ordinarily revealed."

Martin’s slightly younger contemporary, A. A. Hodge (1823-1886), American Presbyterian and representative of "Old Princeton," had this to say on Westminster Confession 25:2:

But our Confession intends in these sections to teach further that ordinarily, where there is the knowledge and opportunity, God requires every one who loves Christ to confess him in the regular way of joining the community of his people and of taking the sacramental badges of his discipleship. That this is commanded will be shown under [Westminster Confession] chapters xxvii.-xxix. And that when providentially possible every Christian heart will be prompt to obey in this matter, is self-evident. When shame or fear of persecution is the preventing consideration, then the failure to obey is equivalent to the positive rejection of Christ, since the rejection of him will have to be publicly pretended in such case in order to avoid the consequences attending upon the public acknowledgement of him.

From all this, it is evident that extra ecclesiam nulla salus is not "just Engelsma’s view!" This is the explicit teaching of several major Reformed and Presbyterian creeds (the Catechism of the Church of Geneva, the Belgic Confession, the Second Helvetic Confession and the Westminster Confession) and the churches in Europe and around the world that have maintained them, Luther’s Larger Catechism of the Lutheran churches and many theologians in the history of the Christian church—including some of the greatest—such as Cyprian, Augustine, Cyril of Alexandria, Luther, Calvin, Bullinger, Beza, de Brès, Ursinus, Olevianus, the Westminster divines, David Dickson, van Mastricht, Van Oosterzee, Hugh Martin, A. A. Hodge, R. B. Kuiper and P. Y. De Jong. Moreover, scholars of historical Protestant theology, such as Heinrich Heppe and Richard Muller, testify with one voice that this is the orthodox Reformed (and Lutheran) view. In this, the Reformers and their successors are following the teaching of the church fathers, as per patristic scholars, such as J. N. D. Kelly.

Keith Mathison summarizes well the historic Reformation teaching (over against that of modern evangelicalism):

Unlike modern Evangelicalism, the classical Protestant Reformers held to a high view of the Church. When the Reformers confessed extra ecclesiam nulla salus, which means "there is no salvation outside the Church," they were not referring to the invisible Church of all the elect. Such a statement would be tantamount to saying that outside of salvation, there is no salvation. It would be a truism. The Reformers were referring to the visible Church, and this confession of the necessity of the visible Church was incorporated into the great Reformed confessions of faith.

It should also be noted that many of the above quotations—especially those of Olevianus, Dickson, Kuiper and De Jong—provide scriptural proof and give biblical arguments to show that "outside the church there is no salvation" is not "just Engelsma’s view" or even merely the Reformed view; it is the teaching of the Word of God! Bound to Join itself makes this point more fully.