Application: How shall we think of church and those in non-Reformed churches
If, as we have said, a strong correlation between the visible and invisible church makes the best sense of the biblical data, then what does it inform us of the church? It means the Visible Church is very important. It is not a gathering of Christians to sing some songs, hear a motivational speech, and thus perform their "Sunday duty." It is not a place for one to exercise one's desire to serve, as if the church is a charity to help give you meaning and purpose in life through your service. No, the Visible Church is nothing more and nothing less than the Institution of God for His purpose and glory, as we see it. The Invisible church is the institution of God as God sees it. The difference between the invisible and the visible church is not qualitative, as in the non-denominational position that the invisible church pertains to salvation and the visible church is the gathering of those who are saved. Neither is it quantitative, as inclusivists would make it to be. Neither is it temporal, as the Federal Visionists would make it (the Visible as the present; invisible as eschatological). But it is "optical," as in they reflect the same reality from different points of view.
The divergence in viewpoints is what allows for the "exception," or the "extraordinary" cases whereby it is possible for true believers to not be in a Reformed church. Since the Reformed churches are made of sinful people, and true believers may have sin in their lives (including intellectual sins) that prevent them from joining a Reformed church, the Visible Church will not fully correlate with the Invisible Church. But since the two ought to correlate, this means it is a Christian imperative for anyone who calls himself Christian to join a Reformed church. To not do so is sin, whether by ignorance or by intention.
Church is important, and joining a biblical church is important, not for salvation, but because obedience to God is a necessary fruit of our salvation. The first application therefore is to ourselves, in that we ought to ensure we are members of a biblical Reformed church. Please note I did not say "activist," but "member." Unless you are called by God, no Christian should feel obligated to serve in the Church, as if the Church would die without your service! The whole purpose of Church is for Christ to feed his flock, not for them to serve Him!
The second application comes to how we ought to discern and treat those outside the Reformed churches, and this is where what I say would be regarded as uncharitable by many professing Christians, but the truth is the truth. If the strong correlation between the visible and the invisible church is correct, then the saying by Cyprian, as interpreted by the Reformed churches, stands true: Extra Ecclesiam Nulla Salus — Outside the Church there is [ordinarily] no salvation. Nice to say perhaps by those who like to sound off the church fathers, but the implication is this: Ordinarily, those in non-Reformed churches should not be regarded as Christians. Let's repeat again for effect: Ordinarily, those in non-Reformed churches should not be regarded as Christians.
This does not mean that we should go around condemning any and every professing believer not in a Reformed church to hell. But this must be the starting baseline. Only from there then we can say that there appears to be many Christians who are not in a Reformed church, and thus we ought to invite them in. We ought to plant as many Reformed churches as possible, and strive to bring people in, converting unbelievers to be sure but also to "convert" evangelicals to join the Reformed church. Many evangelical churches do not exhibit the three marks of the church anyway (Sound preaching of the Word, right administration of the sacraments, proper exercise of church discipline), and they ought to either close down, or repent and become Reformed.
But in order to prevent misdirected zeal, let is be said that the Reformed tradition does not necessarily imply only strict Presbyterian and Reformed churches. The best of the Anglican tradition for example is that of an English Reformed church. There are churches more or less pure, and thus one shouldn't condemn another church merely for being less reformed. The issue is whether the church is Reformed, not how Reformed the church is. This requires wisdom to discern, but the most important thing here is to establish our principles. Does something make a professing Reformed church not Reformed? That depends, but certainly one should be able to say that any church that denies the Gospel is not Reformed, like all Federal Visionist churches.
Ecclesiology sounds abstract and thus appear very theoretical, yet it is actually immensely practical. We have looked at the biblical data and stated that only a strong correlation between the invisible and visible church meets all the biblical data. But if all that is true, then it has immense application for our view of church and church membership, and how we ought to discern other professing Christians. It should make us love the church more, for after all Christ loves the church, and treasure our membership in her. It should also make us desire for the growth of the Reformed church, and desire for all professing believers to believe in the Reformed doctrines and join Reformed churches