Christians usually evade claims of circularity by saying that God told them they could trust the Bible, more or less. Not that they necessarily heard a voice from on high, or anything: when I was an inerrantist, I think I thought the Bible was inerrant because I just felt it was right (there's an old newsgroup posting where I say that, more or less, but Google Groups has lost it: perhaps I'll post my own archive at some point). This is what Christians refer to as the inner witness of the Holy Spirit. robhu and I discussed this the last time Ehrman came up here.