Christians, what would make you wonder or believe "Hey, Maybe the gospels are just somethings some people made up, just like Islam"?

Islamic belief is the synthesis of an abridged/altered Old Testament, plus a number of sixth century, Christian heresies. So, it's not totally "made up".

The fact of the matter: Nothing because God The Lord Jesus Christ our Lord and Savior holy word the bible is truth and man is a liar. Because man is wicked, read Jeremiah 17:9, read Romans 1:17-32, read 2 Corinthians 4:4, read Ephesians 4:18, read 2 Thessalonians 2:3-14

I did wonder that many years ago, but in-depth analysis proved otherwise.

Because of the history of the gospels for starters.

Expecting me to believe four people got together to write "Harry Potter" is way more unbelievable.

Nothing.  It's God's Word. 

The gospels are peoples accounts of Jesus life and teachings