Now, perhaps it is understandable why the early church was "exclusive" and why this is a good thing after all. But that was then and this is now. The church was befuddled by a pre-modern understanding of the world and restrained by obsolete cultural mores, but now we have the benefit of science and have abandoned those outdated paradigms. We have, in short, evolved, grown-up, become an adult church able to seriously reflect on our more childish past. We can now see that the early church was emerging into a largely pagan culture, so a conservative approach was necessary to make sure the church was not subsumed into the larger religious landscape. But now, the church is losing relevance in a post-Christian culture and so must turn to a more progressive paradigm in order to salvage itself and march boldly forward into the future.
Or so the theory goes, anyways. It is true that every generation must reappropriate Christianity and come to express it in terms they can understand. I think the first Ecumenical Council in Nicea is a perfect example of this. By the 4th century, the church had expanded considerably and had emerged from a back-waters offshoot of Judaism in a dirty little corner of the empire, into a more urban, educated and Gentile faith. It had grown large enough that the government had taken notice of it, and not always favorably, and the considerable weight of pagan thought was being brought to bear on the Way. Heresies within the ranks had also started to pop up, and were in serious need of redress. In short, the Church had a whole lot to deal with and needed to be decisive about it. So they have a council, come up with the creed and define the doctrine of the Trinity. However, the language used to describe the Trinity is not biblical and, if memory serves me correctly, has only scant use in patristic texts, so the question is - would the Apostles have accepted this definition? I think it is safe to say that after breaking out a dictionary, maybe a couple of glasses of wine and some prayers, that yes, the Apostles would accept it even though they may not have defined it in precisely those terms.
Every subsequent Ecumenical Council was a similar process of a taking the Apostolic deposit, and re-expressing it more precisely and more clearly delineating the boundaries of right-belief. A similar, though certainly not authoritative in the sense of the councils, process takes place with each generation. The Apostolic deposit and the accumulated understanding of the past is re-expressed, but in such a way that it does not deny that accumulation (except in the case of the Reformation, but that is a whole nother series of posts). The problem with those advocating "radical inclusivity" today is that it attempts to invalidate the understanding and knowledge of past generations. Its basically thumbing its nose at hundreds of thousands of Christians as corrupt, inept or just plain stupid. The RI church assumes it can know either what the genuine Apostolic deposit was, or that the current culture somehow re-defines the content of the deposit.
The perils of the latter are clear. If culture says slavery is fine, then the Apostles did, too. If culture says abortion is cool, then the Apostles did, too. If culture says atheism is great, then...well, that might be stretching it some, but you get my point. The unchanging standard of truth, ie God, is constantly changing based on what our particular society is doing. It also negates any serious moral critique of another culture's practices since culture is the lens through which God is viewed. The former has its own problems. How do you determine what is authentically Apostolic and what is not? How can you be sure that your determination is not being influenced by emotion or bias? Is a doctrine you don't like not Apostolic because its a later addition, or because you don't like it?
The problem with the RI church is not just that it splits from the past understanding of the church, but that it effectively separates itself from the church in history and eliminates one of the greatest treasures available to modern Christians, specifically, 2000 years of the accumulated wisdom, experience and guidance of other Christians. The paradigm that says the past was wrong, and possibly wrong to such an extent it was sinful, really stands as a final divorce from historic Christian thought. It is more than merely saying we need to re-express Christianity in meaningful terms for today - it is saying we need to invent a totally new faith. In forging out ahead, they are actually forging out alone.