You could always become a TV evangelist and say it's gods will to let your little moses enter her pearly gates to the promised land.
Oh and if they get pregnent just say to them to tell their folks she is still a virgin and didnt know how it happened.... chances are they have heard that bullshit story before somewhere.
On a serious note two things:
As Maxovic says why ARE you wanting to sleep with these kind of girls?
If it's your fetish or whatever then If you hang out with them and really express an intense Christian vibe and I mean really really over the top 'love for your god and jesus' and act like this around her you will get her to play along over power her frame and all that jizz er I mean jazz, since regardless of the hobby, favourite past time or profession, women are attracted on some level to someone with intense passion/purpose/drive in that area, be it Art, Music, snowboarding or even Christianity. Women are emotional vampires (in a good way) and they will want to board your fun roller coaster emotional ride, once they do it becomes alot easier. Once you get her hanging off your every word and actions in Christianity, you can start introducing connected intense christian experiences which you can connect to sexual contact. Alot of these types of girls are actually channeling sexual desire into their faith, once you connect their faith and love to being expressed via physical contact you should start to get what you desire, this is a time investment not a ONS.
Also depending on the denomination of Christianity you can talk about it in certain points and express a view which is a 'work around', you just need to think of it from their perspective in the context above.
examples:
1) sex is an expression of love which is in essence the celebration of god and his gifts to all mankind
2) think of sex as a religious experience (not in that way haha) or expression, if you can get that across thats it.
3) Anal is not sex(lol) but a way you can be close and express your love for each other while still obeying your faith and 'not having sex at all' - very good for strict catholics, I used this at uni a fair bit
oddly enough it's the reverse 'oops' while you have her in throes of passion, you might accidentally slip it in the right hole and try give her a few powerful thrusts 
whether there is a hymen to break is another story, if you get to that point and you do this, dont make it a big deal if she does pull you out or ask you to stop, just say oops and do so, then try again later except with the change that you start to 'make the effort' slowly pull out. Chances are she will wrap her legs around your hips and guide you back in , case closed.
4) Treat these girls right and always always make sure you relate the sexual side of your connection as more of an intense spiritual thing than a basic physical thing(even though in reality it is) otherwise they get intense guilt (especially the catholic ones) and feel dirty and will actively avoid you to stop it happening again.
To summerise treat them like normal women to escalate and flirt, focus on religion as your dual focal point, and when it comes to being intimate become like a crusader and use god as your weapon to achieve victory!
p.s if your not a religious person you best start reading up about it.
p.p.s Be very careful you must respect these girls, and handle them correctly, as alot of them will take this experience with them through their entire lives, so you best make it a pleasant one or you are by your desire, actions and to an extent LIES enforcing on them a unfair guilt and other negative emotions - not the usual break up cycle stuff. TBH the hard ones are not ONS's and if your not religious they may not be compatible GF material so why all the effort?
