Isn't it ironic...don'tcha think?

This won't be a terribly long entry, but I just wanted to comment on an interesting exchange I had on a message board with someone of a more Reformed background. As I'm working through all this stuff in my head, I'm engaging Catholics and Protestants in some debate over various differences in doctrine between the camps, playing a little Devil's Advocate to see if I can dig deeper than the stock apologetic responses each side tends to give.

So at one point in the discussion I ask how Reformed Guy knows that his interpretation of a particular passage is the correct one. His response was basically that the Mormons and Jehovah's Witnesses ask the same question and accuses me of saying that we can't know anything that Scripture says since almost every passage in the Bible has more than one interpretation. This is an obvious red herring because I'm not talking about cults that come up with novel interpretations of Scripture or completely new doctrines that can't be found in any writing of the earliest Christians. I'm talking about a passage that, for instance, Catholics take literally and Protestants interpret figuratively or vice versa. How do we know who is right?

Here's the ironic part. He goes on to criticize my appeal to early Christian writings as a typical Roman Catholic apologetic tactic then tosses out this little barb at the end:

"It is clear that the Catholic church says they do not teach or practice idolatry, but it is equally clear that they do in fact teach and practice idolatry. Of course in the post-evangelical mindset, we forget history and adopt a post-modern approach to defining what it means to be a Christian."

So help me out here: when interpreting Scripture we shouldn't appeal to the writings of the earliest Christians, many of whom were contemporaries or direct disciples of the Apostles to help us understand how to interpret difficult passages because "that's what Roman Catholics do", but at the same time our problem in not interpreting Scripture correctly and understading what it means to be a Christian stems from forgetting history? Which is it? Do I look to Christian history as a guide and context for understanding the Bible or not? Or is it that only history before 90 A.D. and after 1517 is legitimate to appeal to as a guide for interpretation?