Two years ago, I was at an apologetics program put on by Campus Crusade featuring Dr. Norman Geisler. The presentation was fairly basic, and the contents familiar to most of us here. But, I stuck around because I had a burning question and there was the promise of a moderated Q&A. So, I submitted a written question - without resorting to a Magesterial argument, how can someone know what the contents of the New Testament canon were? How do we know that James belongs, but the Didache does not? Dr. Geisler hemmed and hawed, and gave an answer about manuscript evidence, but never really answered the question - leaving me with the inescapable conclusion that one of the foremost Protestant apologists of his day didn't have an answer.
For a while, that really bothered me. Now, I accept that there is no answer - and have to accept the Church's Magesterial authority in determining the canon.