Just discovered this site, and I think it's wonderful.
Just a bit of a quibble though. It seems to me that 'Baptism of fire' probably has its origins in the Gospels, doesn't it? Somewhere in John 1, 'He will baptize you with the Holy Spirit, and with fire...'?
Also, I would like to suggest a 'Phrases of Biblical Origin' to complement those of Shakespearian and Nautical category.