Man-made sounds bad. Can there be anything more man-made than doodlings in a cave? or the writings of the early Fathers who may (1) just giving their own fallible thoughts (2) whose writings may have been altered or modified or (3) completely spurious fabricated writings from an unknown author. Or is there anything more man-made than the and the liturgical and devotional practices of the churches which have no Biblical basis? Arguing against the "man-made" road is only going to work against your cause.
Prov. 30:5,6 Every Word of God is pure, He is a shield unto them that put their trust in Him. Add thou not unto His Words, lest He reprove thee, and thou be found a liar.
The Muslims say that we are "People of the Book," but we are not that (only): we are People of the Word.
Because the Word is not only what is written. The Word is first spoken, then performed. Then written.
This is certainly true of the NT, which was spoken and performed for decades, -- parts of it for generations --- before it was written. If you just take the written part, precious though it is, you are missing out on a lot of the Word.
Without the "spoken" word --- the Oral Tradition, the Preached and Practiced Tradition--- we wouldn't even know what the books of the NT Scripture are. We wouldn't even know who wrote the Gospels of Matthew, Mark, Luke, and John, since their names are not listed as authors of these books. No in the manuscripts! We know it only by the other avenues of Sacred Tradition.
And--- inasmuch as the written Scriptures were transmitted, collected and canonized for us by the Fathers of the Church, why would you trust the accuracy of one part of what they handed down to us, and not trust the rest of what they handed down to us?
It was the Oral Tradition that birthed the Written Tradition.