I heard it was something to do with the cross?

( whenanimalsattackhumans clad in global hypercolour, Mon 24 Aug 2009, 13:43, 1 reply, 16 years ago)

Nein.
Twas due to the Latin number, X, because the letter that begins the Hebrew spelling of Jesus was the tenth in the alphabet. I forget the details, possibly it was 'Isvs' with I/J (they were the same letter in ye-Latin times) being the tenth letter of the Latin alphabet - or possibly it was the Hebrew equivalent of that.

I am an exciting man.

EDIT: Wikipedia has just proven me wrong. Go with the Greek explanation, it's better.
(Land of Green Ginger It's dreamy weather, Mon 24 Aug 2009, 13:54, Reply)

I knew about I and J
but never twigged they were next to each other thus being the same letter.

I'm a bit dense, I should have noticed that!
( Lampito rise with the moon, go to bed with the sun, Mon 24 Aug 2009, 14:04, Reply)