My girlfriend just told me she use to fool around(sexually) with her best friend up until 1 one year ago?

My girlfriend just told me that she used to fool around(sexually) with her best friend up until 1 year ago, when we started dating. I'm not gonna judge her for things she did in her past but I'm a Christian and that bothers me a bit. I love her to death and want to spend the rest of my life with her but why come out of the blue and tell me this now. Even though she said they haven't done anything while we've been together. Who knows they could still be doing it because she is very close with her best friend. I just don't know how to handle this right now =(