Midcourse corrections

A stock Catholic objection to Protestant theology is that you can't find Protestant distinctives in pre-Reformation church history. For instance, you can't find sola scriptura or sola fide in the church fathers. Suppose we grant that contention for the sake of argument. How does that disqualify Protestant theology?

i) Truth isn't determined by taking a headcount. The correct interpretation isn't determined by taking a headcount. Many or most folks believe much or most of what they do due to social conditioning, which is one reason appeal to consensus is a highly unreliable guide to truth. 

ii) Popular opinion carries no presumption that the opinion is true. For that matter, mere scholarly opinion carries no presumption that the opinion is true. An interpretation is only as good as the logic and evidence adduced in support of the interpretation. Reasons rather than opinions are what merits consideration.

iii) Whether or not people find an argument convincing is irrelevant to whether it's a good argument. Atheists find arguments for Christianity unconvincing. That, in itself, doesn't invalidate the arguments for Christianity. Most Jews are not persuaded by the messianic claims of Jesus. But that, in itself, doesn't make the messiahship of Jesus doubtful. 

iv) It's also the case, in the history of ideas, that an erroneous idea may not appear to be erroneous at the outset. Or it might seem to be a minor error. We may only come to recognize the error, or appreciate the magnitude of the error, as it works out in practice. As people begin to build a political or theological edifice on that seminal, seemingly innocuous idea. Marxism is appealing on paper because it's utopian. The irony is the chasm between how idealistic Marxism is on paper and how inhumane it is in reality. 

Likewise, certain theological ideas involving the status of Mary or the nature of justification (to mention two examples) may have unforeseen consequences until those undergo further development in theory and practice. At that juncture it's easier to recognize where things got off to a bad start. 

The Catholic doctrine of justification gets bundled with other things like Purgatory, indulgences, and the treasury of merit. Likewise, Catholic Mariology begins to usurp the prerogatives of Jesus. Consider, too, the role of Marian apparitions in popular Catholic piety.  

Seminal theological errors have a snowball effect over time. At that point Protestants may bypass long stretches of historical theology and go back to biblical revelation because the development of dogma went offtrack, and that's easier to see in hindsight. That's a stimulus to reexamine traditional Catholic interpretations of Scripture. Something went wrong early on, requiring a midcourse correction. For Catholic apologists to complain that these are theological innovations misses the point. If Catholic traditions are a source of error, they need to be replaced. Like installing a new car part to replace a defective part. Redesigning the defective car part. If Catholic theology suffers from design flaws, and that becomes increasingly evident after the fact, then fixing them after the fact is necessary. And repairs will take place at a later stage in church history because the historical process exposes the design flaws.