from:
http://www.modernreformation.org/mr98/julaug/mr9804sixth.html
a very good article, you ought to read it long before you look at my scribblings.
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A Sixth Sola?

By John R. Muether

Extra Ecclesiam Nulla Salus. There is no salvation outside the Church. In the good old days of American religious warfare these were perhaps fighting words for many Protestants, as they smacked of the mysterious and repressive haughtiness of Catholic sacerdotalism. Today, claims of the Church's exclusivity seem quaint and inconceivable, not least among Roman Catholics themselves, who are given to speak of even atheists being "anonymous Christians," and Eastern Orthodox and Protestant communicants as "separated brethren." Such incredulity testifies to Americans' ignorance of church history, because the statement goes back to the ancient Church. Nor are such claims the exclusive property of Rome, because they were frequently invoked by the Reformers. Beyond historical illiteracy, such claims' incoherence betrays the biases of our anti-ecclesiastical age.
...
Simply put, there can be no Christian life apart from the Church, according to the Reformers. No one can come to faith alone nor live by faith alone. Our faith is not from the Church, it is a gift from God (Eph. 2:8). But it comes through the Church, through whom the wisdom of God is made known (Eph. 3:10).

But what did the Reformers mean by the Church? It is rightly claimed by low church Protestants that the Reformers developed the distinction between the visible and invisible Church in part to refute the sacerdotal claims of Catholics. The invisible (to us), universal Church is "the whole number of the elect" from all ages (WCF 25.1), the "church of the firstborn, whose names are written in heaven" (Heb 12:23). The visible Church consists of confessing Christians and their children (WCF 25.2). The latter, of course, contains sinners and hypocrites, and is thus always, in this age, an imperfect embodiment of that Church visible only to God.

This distinction is often misunderstood, and contemporary interpreters in evangelical circles make more of it than the Reformers intended. The Reformers never suggested that the visible Church was of little or no importance. As the manifestation of the invisible Church to the world in time and place, the visible Church, though imperfect, remains the true Church, because it displays the marks of the Church. And it is the only Church that we can see and with which we can have fellowship. We have no Gnostic recourse to any other church than the visible Church.

from: http://www.fpcr.org/blue_banner_articles/visible1.htm
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The Westminster Confession of Faith XXV:2 claims as much for the visible church, where it states, “The visible church, which is also catholic or universal under the gospel (not confined to one nation, as before under the law), consists of all those throughout the world that profess the true religion; and of their children: and is the kingdom of the Lord Jesus Christ, the house and family of God, out of which there is no ordinary possibility of salvation.”

two excellent essays on the topic
quoted from an essay i wrote at:
http://www.livejournal.com/users/rmwilliamsjr/56767.html
on my problems on joining the church.