Religious folks (at least the majority in the US) believe evolution is false. A microorganism in Mars won’t change their minds. I actually suspect no amount of evidence will.
1) What do you mean by secular authority? What do you mean by religious authority?
2) Why would anyone use the same arguments theists make? Such are logical fallacies to begin with.
3) Science and reason are valid because they work, bitches.
The parents are obviously hostile to your lack of beliefs. First thing you will realize is that no matter what you say or what do you will not change their minds. You can’t reason people out of something that they did not reason themselves into in the first place.
You can either respectfully decline to participate in their religious activities or be just as hostile with them as they are with you. The latter will get you in trouble with the GF, the former requires a lot of patience (and I mean a lot) to do.
Personally, I refuse to spend time with people that are hostile to me. I don’t and I won’t put myself in a situation where ignorant people can try to force anything on me. If the GF can’t understand that then I will refuse to spend time with her too and call it off. The older you get the more you realize that you can’t put a price on peace of mind.
That’s the beauty of the Bible - one single phrase can mean 12 different things depending on who you want to fool/teach/curse/hate/love. It’s like the ultimate rorschach test.