Spelled out phonetically, it's "'kris-m&s" (M-W).
Yet, the root word Christ, is pronounced "'krIst" (M-W).
I don't know anyone who says it "'jE-z&s 'kris", do you?
Merriam-Webster clears it up:
Etymology: Middle English Christemasse, from Old English Cristes mæsse, literally, Christ's mass
I'm thinking that in the hustle and bustle of the season, compounded by the increasing trivialization of the event, has led us to change the meaning, and subsequently the pronunciation, of Christ-mas. It hasn't truly been a celebration of Christ's birth in a long time - the commercial aspect has long dominated the day.
So here's my plan: Let's fight back.
Let us say Merry Chris-mas no longer.
From now on, it's Merry Christ-mas.
And bah humbug on anybody who takes offense.
Merry Christmas, y'all.