I haven’t had a chance to look much at this site, Regensburg and the Spirituali, but it promises to be interesting.  Much of the Diet of Regensburg was a non-starter between the Roman Catholics and the Reformed and the Lutherans (Melanchthon).  But on justification they reached a formula that with which all could agree.  Calvin wrote to a friend:

Our friends have thus retained also the substance of the true doctrine, so that nothing can be comprehended within it which is not to be found in our writings; you will desire, I know, a more distinct explication and statement of the doctrine, and, in that respect, you shall find me in complete agreement with yourself.  However, if you consider with what kind of men we have to agree upon this doctrine, you will acknowledge that much has been accomplished.

Of course, both the Pope and Luther freaked out and dismissed the document.

Really though, it wouldn’t have mattered if the had accepted it.  There were still great issues that divided Christians.