Frank Turk raised a question about the objectivity of the covenant and emergence, but at least had the presence of mind to realize it was a baptist question. This is not a detailed answer to the question, but it provides the initial outline of an answer.
I believe emergent errors need to be identified and opposed, just as I believe Roman Catholic errors need to be identified and opposed. This is a separate question from whether or not emergents or RCs have a covenantal obligation to abandon their errors, which they do. If they are baptized in the triune name, then they have an obligation to follow Jesus in true truth.
Now suppose someone came to our church from Brian McLaren’s church, baptized by McLaren himself, and requested membership here. Would we require rebaptism? The answer — in line with the objectivity of the covenant — is absolutely not. I do not deny that McLaren has the authority to perform valid sacraments.
I will return to my standard illustration, which is that of the covenant of marriage. A man who is married to a woman is obligated to be faithful to her. But if he is not faithful to her, this does not mean that he is not “really” married to her — because that would mean, ironically, that he was not really being faithless to her. You have to be covenantally obligated to be covenantally faithless.
People who chase after various winds of doctrine should stop it, regardless of what they are. They should remain true to the faith once delivered to the saints. But if they do not stop it, this does not change the objectivity of their obligation. So, to take an easier case than McLaren, let us consider an overt infidel who happens to be baptized. He is a Christian in one sense (in terms of the objectivity of his covenant obligations) and not a Christian in another (because of his faithlessness).
If a cheating husband repented and came home to his wife after years of infidelity, and she forgave him, and said to him, “Today, you have become my husband,” we would all know what she meant. He had to have been a husband before his repentance in order to cheat, but when he repented of cheating, he “became a husband.” A lot of faithless covenant members have “become Christians” the same way.
A lot of the problem is caused (in my view) by the evangelical absolutizing of the noun Christian. But the word can be used in more than one way.