I think that it is important to dicern what the major difference is between the Protestant understanding of docrinal development and the RC understanding, so that we can see how we understand the development of doctrine differently. For both Protestants and Catholics we find “seeds” of our thought throughout Church history. It is in the justification of these seeds that the differences emerge.
Protestants would see the legitimacy of unspoken tradition as somewhat reliable for a time, but subject to corrution that all unwritten information is subject to. Therefore, a Protestant might be more willing to accept the concensus of the early Church’s beliefs when the early Church fathers attribute these belief specifically that which they recieved from the Apostles. But these are few and far between (and very undeveloped . . . i.e. theologies of the baptism and the eucharist are very simplistic).
Protestants also see the development of doctrine proceeding when conflict arises that makes it necessary to search for the correct understanding. This search sometimes would reference “apostolic tradition” (only infallible to the degree that it accurately represents the teachings of the Apostles) and Scripture. But for the most part, in the early Church, Scripture was the primary reference because of the inherent untrustworthly nature of tradition and because of the fact that it cannot be varified when it is not written (who is to say who has the right tradition?).
This worked out well when defining the Trinity against competing models (Arian, Modalism, etc) and when defining the nature of Christ (Nestorian, Eutychianism, Apollinarianism, etc) becuase the Scripture speak clearly about these issues. Therefore, the early Church could refer to Scriptures for their primary support, and then refer to “apostolic tradition” for secondary support. Protestants have no problem with this method.
This worked in the early Church (pragmatically speaking) to define the essential orthodoxy of who God is. It worked because the the controversy occurred within reasonable time proximity with regards to the apostles testimony (100-250 years) and because there was sufficient Scriptural data to justify such claims.
Once “tradition” had “settled” the case with the Trinity, it seems only understandable that the Church began to see itself as an “infallible” gardian of truth.
The problem arises when issues and doctrine surface that do not meet these two criteria: 1) to much time has passed for us to rely upon unwritten tradition, 2) Scripture does not speak sufficiently to a matter. Therefore, when issues such as Marian dogmas, savific nature of seven sacraments, and Papal infallibility arise, Protestants do not see them as legitimate “developments” in doctrine because they do not meet this criteria.
Issues such as the Atonement (developed in the 11th century) do meet the criteria because they rely less upon unwritten tradition, but upon the testimony of Scripture alone. The Scripture do speak sufficiently to tell us that the atonement was a price paid to God, not Satan. So this is a legitimate doctrinal development. Other issues such as the faith alone as well do not rely upon unwritten tradition, but upon Scripture alone (although it can be found in seed form in the proceeding centuries). The doctrine of Sola Scriptura is a necessary outcome as unwritten tradition becomes untrustworthy. While the Scriptures do not speak of tradition being theonustos (God breathed), they do say that they themselves are theonustos (although this does not “prove” that they are, but is an arguement post facto that is sufficient for this discussion). Therefore, sola scriptura is a necessary argument from deduction. The unwritten tradition of the apostles is almost impossible to validate, but the written testimony of Scripture is not. Therefore, the Scripture is our only reliable source and the only sourse that is God breathed.
Now, even today, as issues arise, we can study the Scriptures (and written tradition as a secondary source) for the truth. We can continue to develop in our understanding of these things. So docrinal development continues today. This is the principle of semper reformanda (“always reforming”).
This model is not at all unreasonable. All we are saying is that we may know more now than they did then because we have had the time and the circimstances to study issue more carefully. I don’t think there will be any major changes (since the Scriptures are our guide), but maybe better articulations of doctrines already “established.”
Hope this helps to understand the Protestand understanding of docrinal development.
Michael