wow i'm sorry to here that.
but i went through and am still going through something similar. my boyfriend is my best friend and my everything. but my family doesn't approve. the only thing i can tell you is to perservere. parents are tough but you should try to facilitate a completely open conversation with no arguments allowed with them and talk about how you feel. if they aren't open or accepting after that. it makes me question how Christian they really are. Christians are supposed to be loving and accepting. Not saying that the should facilitate your behavior but they should atleast respect you. I say this not to affend but to voice my opinion. wen i came out to my parents about being bi, they gave me a speech said they still love me and prayed. maybe your parents should take the same approach and you take the same for them to be open to you and your girlfriend's partnership
best of luck