Sucks your mom didn't understand, but I'd have to say let it lie. My cousin's family still tries to control this particular cousin with religion (and is doing a damn good job at it) & the cousin is nearly 30 years old. I mean, here's a 29 year old who can't date person XYZ because, "They're dangerous! XYZ doesn't believe in the Lord, so there's nothing holding them back from murdering you! They aren't afraid of hell!". Yeah, this was said to an adult person with a serious face. More than once. On different occasions...yeah.
Also, my mom has came at me with a knife more than once over living my life in a way she didn't much care for, pitched unopened soda cans at my head for just going out with friends after work (I'm 27 btw), even threatened me with a tire iron when I had an auto accident, 'cuz, ya know, accident implies "calculated action" in her world when it comes to me. I'm sure she'll do half of this shit again before we're dead; I'm sure she's got new crazy cookin' up for me as I'm typing this, she's just got to wait for me slip up. I just get over it because she's my mom. It hurts when a parent isn't on board with something so near & dear to the very core of your being (I'm thinking your spirituality/religion is a pretty big deal to you as it is with most folks), but she still loves you & only said she said out of genuine concern for you. Think what you'd say if Odin, for example, said, "Dude, if your mom's not on board with me before her time's out, I gotta make her live with the frost giants. Them's the breaks." I'm sure you'd try to convince her because nobody wants their mother in that frozen wasteland. I hope that didn't seem offensive, none whatsoever intended, just needed a rad example.
It was a well meaning comment that just didn't quite have the effect she thought it would. It sounded like she was holding back too, trying to use a delicate touch-I gotta say, if she's as big into Christianity as you say, that was a classy way of maneuvering on her part. Talk it out, hug it out & I love to top stuff like this off with tasty homemade waffles...I can post the recipe if anybody's interested. Seriously, no syrup needed, these things are flippin' delicious & courtesy of my mom's crazy brain. <3