Believe me I know what you are going through.I have been married 35 years to a non religious believer, and I still get mad at times when discussing these matters so unfortunately you must get used to it . The best advice I can give you is if you really love the person this should definitely not be a deal breaker .Love is more important than a persons belief in a God .When you hear that people break up over these issues it’s probably because Love was lacking on one side or both

My husband and I have had times where we didn’t agree on religious things. He came from a super religious family and I did too, but the difference was I basically pushed it all away. After a while he started agreeing with me but when we disagreed I did my best to keep calm because fighting over it would just cause both of us to dig further into our opinions and not see the others side. Try to keep calm when discussing it and ask the same of your significant other. Sorry I’m not more help.