NTA, but I think you should pick and choose your battles, and this hill is a really small one to die on. There's too much at stake when it comes to the workplace. You could get your way in having religion kept out, but you run the risk of having everyone think of you as the asshole, or even worse, experience retaliation. Also, I know from experience that the harder you try to fight an evangelical's proselytizing, the harder they try to thump their bibles at you.

If it helps, try viewing her as a toddler and Jesus is Santa Claus. Yeah it's annoying to bake cookies every year for that jolly red asshole but it's only one day a year and someday they'll grow out of it. And if they don't, at least they'll die eventually.