As Muslims, we believe that the Torah and Bible were corrupted by man, but that the Quran has not been, as it is under God's protection. When debating with non-Muslims, they find it ridiculous that we believe the Torah and Bible have been corrupted, but the Quran has magically remained untouched.
1) How do we know the Quran is under God's protection for sure?
2) If the Quran is under God's protection, why did he choose to protect the words of the Quran from corruption and not do the same with previous holy texts?