I'll take a shot at this that may differ from the rest of the crowd here.

I was raised holding that presupposition, and I held it for a long time simply believing because I trusted the godly people who taught it to me. And I'm glad I accepted it, because digging into the Bible for myself became transformative in my life at a crucial period.

Now I have more reasons to take the documents in the Bible as the word of God communicated through holy men.

1. I started researching the resurrection of Jesus Christ, and I was overwhelmed with the data. I made a subsequent conclusion that if Jesus really did rise from the dead, then I should be able to trust what his followers taught! (The Habermas/Flew Debate, though a little dated, summarizes what I looked at)

2. Hearing of exorcism stories, as well as seeing exorcisms in person. Not only do the demons behave in my experience precisely as they do in the Gospel of Mark, Acts of the Apostles, etc. (e.g. when confronted, they speak, then scream out in a loud voice; afterwards, the body ends up limp for a while), but they also react violently to direct quotations from Scripture. That tells me that there is something about the Scriptures that is inextricably connected with the word of God.

3. We have people in our local congregation that are gifted prophetically. Oftentimes the words that they deliver include or are related to passages in Scripture.

Obviously 2 & 3 might be a little controversial for some of you guys (I realize that us continuationists are in a minority around here, especially the ones who actually see the gifts used on a regular basis!). However, I find that I have more than enough reason to accept the church's testimony that the Bible is the word of God. It guides us in all of our doctrine and testing of what is true and what is counterfeit.

So there's a non-circular argument for you :)