He has no right to comment on god bless, that's ridiculous. Also its no business of yours if he takes the lords name in vain.
If you have been told not to say it by your manager then stop saying it. It's ridiculous but some times in work you have to ridiculous things.
I am an atheist and I wouldn't even think to be offended by that. Religion is nothing to do with work so I wouldn't bring it in as he is doing.