I think you have to move beyond being affected by what religious people think and say. I'm not affected by their views because my mind is made up. They couldn't even come up to me and try it if they wanted to. I am also ok with not trying to change their minds either. When I talk about atheism publicly, I do it for the people who are questioning religion and trying to leave, not for those who believe it is true.

The Truth hurts. Her foundation was based on lies. You can separate for her that there is a God outside of made up religion. God existed before the Bible was written and if she seeks it, she will find it. The easy answer is God is always there and you are a part of God. You just need to connect with God.

I suspect these people also have doubts and they are looking to others to be a crutch and support their ideas. It might be interesting to try and say you feel she has a right to her beliefs just as you have a right to yours. If she doesn't push her ideas on you, you won't push yours on her.

Getting into arguments sometimes only exacerbate the problems so one needs to find a tactic that is non-offensive yet effective.

My late partner's daughter was always pushing her to go to her church. It always led to an argument. My partner finally realized this was more about her daughter's community not her faith so she agreed to go on one condition (our marriage was full of "one conditions" ). She would go but she would not hold her tongue. When others said merry christmass she would say happy holidays and other things. The daughter was happy.

I also respect everyone's right to their own beliefs. I will defend anyone's right to his or her belief. i will also go off on anyone who refuses to respect my right to my own belief. It was wrong of her to threaten you. They claim that their god gave us free will, but they refuse to let us exercise that free will. Theists are always trying to convert people and force their beliefs onto others. Atheists and agnostics do not go around trying to convert anyone.