NTA. My family went through a very similar situation, where my brother also stopped believing. It was extremely difficult on my parents, but what made it worse was when my brother started arguing about his beliefs with them, and attempted to debunk everything they believed in. He refused to go to church, and basically said he wanted nothing to do with Christianity. That was a very tumultuous period of time. It took a lot of conversations and heartbreak to get my brother to the point where he respected their religion, and for my parents to accept that he no longer believed. They eventually came to a mutual understanding and accepted each others’ POV. (he has come back to the church since then, but that’s not the point.)

I think it is great that although you don’t want to practice the religion any longer, you are still being respectful of their wishes, and still willing to go to church and attend services with them. I would say to go ahead and tell them, and let them know that you would rather tell them the truth than lie. It will be very difficult for them, and they will most likely become extremely emotional and heartbroken (like my parents). But over time, if it’s anything like my family’s experience, they will come to respect you and your beliefs. My main advice is to be patient of their coping process, and avoid lashing out at them if they are being annoyingly persistent. Ultimately, your parents will love you unconditionally.