My fianc?e [27F] is threatening to end her relationship with me [32M] if I don't "stand up to" my parents?

We've been together for 3 years and engaged for a couple of months, I'm going to call her Liz for this. I'll admit that I have let my parents get away with a lot of comments I should not have that have caused Liz a lot of unnecessary hurt, but they have a mostly good relationship. She says she feels that she has to mask who she is around them and has accepted that when we have children my family will hate her for being unwilling to push religion on them, which I have tried to tell her won't happen.
We are planning our wedding now (mostly her). Recently we had a discussion on location and I mentioned that I think a lot of churches are pretty and probably cheap. Liz vetoed it and went on to mention that she's unwilling to have a religious wedding and realized that my parents will probably be upset. She then said she thinks she would like to elope as her family is fully supportive (they've already watched her get married to her late husband) and she wants the wedding to be for us and what we value, not about sending a fake image that will inevitably be broken anyway. I was hurt and told her she was being unfair to my family, and that we would need to find a compromise.
Liz told me that we either elope or I set boundaries with my family about what is acceptable to complain about. She brought up a conversation with my mom where she had gone to a funeral with no religious references and had talked about how wrong that was and "how can you have any hope at all that they went to the right place otherwise?" Which, Liz as a widow ended up privately crying about and I did defend my mom's right to have an opinion which I kind of regret.
I told her I love my parents, and that they care about her despite what she thinks. I said even a small religious reference would be enough to appease them without compromising her values. Usually Liz goes out of her way to be kind and respectful to everyone, but she told me that maybe I should find a different bride if that's genuinely how I feel and went home. What can I say to her? I love this woman, but she's come into the relationship knowing my family is like this and that they will be devastated knowing I deconverted.
tl;dr my fianc?e has put up an ultimatum of me telling my family I have deconverted and we will not be having a religious ceremony for our wedding, or privately eloping. I don't like either of these options and I feel like she doesn't understand what kind of position this puts me in.


Source.