Anyone who is "frequently trying to convert me" either isn't willing to respect my point of view or is pulling a "Blues Brothers" (on a mission from god). Either way, it would get old with me REAL FAST. Pointing out the fact that (I assume) you have NOT been disabusing her christianity and keeping your peace while she has been playing proselytizer might get her to reassess her attitude.
Life is too short to put up with a constant irritant. If that is what she wants to be, I would heartily suggest you take a walk.
My wife is a Christian (though she hasn't attended a church in years) and says she's never hated anybody enough to call them an atheist. We've been married for 57 years. But from 2000 to 2006 I was president of the Humanist Society of New Mexico; the newspaper did a small feature with pictures on me. What we have is worth keeping, but there are difficult moments. But she never tries to convert me (to get me back in the fold, it would be; I was on the board of a Presbyterian church when I realized I was a humanist), and I never demean her beliefs in her presence. I couldn't continue if she kept trying to change me.
I'm with Jerry. My wife is still a theist, and we have many difficult conversations to navigate, but proselytizing has never been a part of our interactions.
You aren't going to want to hear this at all, but I strongly advise against taking this relationship any further. It doesn't sound as though this is going to dispense on her part, and trust me: it will get MUCH more annoying if you marry this girl, especially if she then imposes her forward christian beliefs on any children you may have (which you can be entirely certain she will).
First the general question. When I'm confronted by someone trying to 'convert' me I start with a smiling, respectful, 'yes, OK', and often they stop there. If they persist I tell them that I am not at all religious, that furthermore I believe in no gods, and that their efforts are wasted on me and not a little annoying. I always offer to continue the conversation because I really do like to hear what others profess to believe. I try to stay non-confrontational, but sometimes it just can't be helped if the conversation is to remain honest. I find that each time I move to a new location the Jehovah's Witnesses come for about a year and a half and then give up.
A proselytizing mate is another kettle of fish. My Dear Ex is very religious, but seldom tried to convert me except with very weak attempts that she never really expected to succeed. Likewise, I didn't try to disabuse her of her beliefs, but would answer honestly and non-judgmentally when asked. I wish that I could report that our diametrically opposed philosophies balanced and reinforced one another. Well maybe they did at times, but after 25 years they proved insurmountable and we finally split.
Couples needn't be perfectly congruent, and it can be a good thing when they are not. But core belief differences probably will remain differences, and likely prevent a lasting full connection.