While there is always a more diplomatic way of handling any situation, hindsight is always 20/20. If you had politely declined her invitations repeatedly and she still asked several times more, I think you did what anyone else probably would have done. You are only responsible for your own actions and feelings, not hers. Diplomacy only goes so far sometimes and then you have to be more direct. What I see is her being in the wrong but not being able to accept responsibility for it. I don't see how you insulted her beliefs when she clearly did not respect yours. Given the situation, I think you handled it OK.

Speak softly, but carry a big stick, is something I rather enjoy doing.
I get what you mean though.
I have a lot of friends that are believers. but for the most part we never have any issues about what each other believes.
But yes, I have had some so called friends, that absolutely refuse to stop trying to convert me.
It's succinct to say that most of the latter, are not friends of mine any more.

She has apparently picked you to try to convert you to her church. No, there's no polite way to say no, because she won't accept no for an answer. She's using the manipulation techniques that the church has taught her. Yes, they actually talk over strategies for converting people. Her snit was part of the act, so don't feel bad about it. You didn't try telling HER what to believe, after all. Not following someone else's religion is not insulting them.

I heard a good comment recently that when Santa Claus lands on my roof I'll believe in him. Well, when god personally appears out of thin air and invites me to a church I'll go.

Of course you could have. But diplomatic doesn't mean better. Sounds like the softer approach has not worked, and you did what you need to do. This is about you and your feelings. If your acquaintance takes that as an affront, so be it.

In this case, who cares about diplomacy...and why should you have to defend yourself and your beliefs? She is the one who tried to push her religion onto you!

That drives me crazy. Here they are disrespecting your beliefs and if you respond you are disrespecting theirs.

Probably leaving out the opening question would have been better, since it made things personal for her. & then saying you don't like the preaching part because, as she knows, you don't share her beliefs. But if she's been insulting your beliefs for years, maybe it needed to happen the way it did.

Many years ago I worked with a Roman catholic guy and one friday I was sitting in the works canteen tucking into my steak and chips (I think it must have been payday) when my Catholic friend joined me with his fish and chips and then started berating me for eating meat at his table on a friday. He was quite a bit older than me and very kind to me so I just laughed it off.
It is always a touchy subject but if your friend is a christian surely she should forgive you . Of course maybe she feels that it is her duty to save your soul from eternal damnation and will nver let up.