[quote=GordonBOPS]The response I’ve had to that very question is “Because the Holy Spirit tells me so.” I don’t know what to say to that. I’ve also had the general response: well, God made sure it happened right with scripture, because it had to be that way, otherwise, the Church has no special authority.
This is the same argument the LDS use to “prove” the inspiration of the Book of Mormon. Assuming your friend is not Mormon, you might want to point this out.
Also, you can ask them if they have read all ancient literature. Did they read the Didache to see if the Holy Spirit would tell them if it was inspired or not? What about the Gospel of Thomas? The Shepherd of Hermas? Clement I & II? etc. etc. You can further this by asking them about the OT books - Leviticus, Numbers, Deuteronomy, etc. are pretty dull and most would agree they are hard to find inspirational.
Finally, ask them how come the Holy Spirit is guiding different people to regard different books as canonical. Is the Holy Spirit guiding people into contrary positions? Pretty soon, the Protestant is either force to concede that they must rely on the Catholic Church’s decision, or they must claim that they have explicity authority by God (that no one else has) to decide the matter for themselves. That 2nd one is usually a pretty hard sell.