Most Helpful Girl
Go to church! To address her reasoning: everyone is a sinner - that's the point, isn't it? That's like saying sick people aren't allowed in a hospital. Your grandma is trying to make a point to you and trying to get you to stop doing stuff. I can definitely understand where she's coming from, but she's wrong here. You shouldn't just be going with him to hang out or for social reasons obviously, and that's what she may find offensive. If you are genuinely going to church tho for yourself and you wanted to be kind to her, you might just go to your own church and not go with your boyfriend to his church.
BUT you could make the argument to her that sharing religions, making sure you have compatible beliefs, and building that bond as people who support each other theologically, is one of the most important things in your relationship. It's very important to get to know how he treats his religion.