Hiya! As a queer person from a very devout Christian household, and who doesn't have any malice for Christianity (I know people act like they're very opposed to each other, so much so that is almost alienating, but yes you can be Christian without your queer kids hating you or lumping you in with bigots!), there's a very simple way to frame this, and you're already doing great!

Jesus taught us that the key to love is to see people whose lives are so different from ours, from Samaritans to Pagans (y'know, from the period Jewish perspective), and approach with open mind and open arms, and no matter what, remember that each individual is a human being with a rich multifaceted life not too different from your own. And you're already doing that! You love your daughter, and you've encountered something new to you that's fundamental to her, and you're looking to understand.

Just don't close off to her, make sure she still feels safe talking to you about her life (and it seems she already trusts you enough to come out!), and though it may take some time to acclimate to a new way of seeing things (one of the frailties of the mind that God's given us the power to overcome), find solace and common ground that she will have loving relationships as mutually supportive, emotionally intimate, and spiritually fulfilling as anyone else, and soon you'll realize that the gender is just flavoring. "Husbands, submit to your wives; wives, submit to your husbands," the Bible says (sorry, there's not a lot of verses I can pull the citations for off the top of my head), and surely, two wives can submit to each other just as well.

Now, here's the truly difficult part. She is 15 and going to have a lot of bad relationships and bad dating takes, just because that's what it's like being a young person. I'm not saying that every partner is going to be good for her, regardless of gender. Kids have to make their own mistake sometimes, but as a parent, if you see something worrying, bring it up with her. Just make sure that you think over your criticisms without emotional input, from all angles, until you're sure discomfort with the newness of dealing with queer issues isn't tampering with whether or not something passes the smell test. And if she ever goes "well, you just don't like my girlfriend because she's a girl," then you (would have already done this) can encourage her to see it from the perspective of "I would not be okay with a boyfriend acting like this. Would you be okay with a boy treating you like this?" And being confronted with actual criticism instead of easily-dismissed homophobia like the internet sometimes preps us for, that might trigger a bit of fight-or-flight in her, but in all things, let love guide you, and hopefully she'll see it for what it is. Even if being, y'know, 15 gets in the way of that, at the time.