This is how I would look at it, and deal with it if I were in your place.
First, unless my parents were known to regularly have terrible judgment, I would trust their judgment about my grandfather’s driving ability, since they have a lot more experience than I do both with him and with driving.
Next, I would be ok with missing Mass, if he is my ride. The church does not require you to get to Mass if you can not do it safely. People who live in areas where there are bad snowstorms, are known to skip Mass if the driving may not be safe due to snowy roads. This would be the same type of situation. Given the fact that he is not likely to become a better driver, I would start looking into other ways to get to Mass, and if possible, other ways to help my grandfather get to Mass as well - whether that means calling the church and asking for someone who you could contact about driving you, looking into bus routes, or working towards learning how to drive. I’m not clear on what your exact situation is, since your parents can’t get you to Mass but your Grandfather can, so there may be other options as well, depending on your personal circumstances.
And last, I wouldn’t want that confrontation with Grandpa either. If it was possible to avoid it without lying I would. But I’m not sure if you’ll be able to avoid it for very long, especially if he is going to expect you to get a ride from him every week. Personally, I feel pretty strongly that if your parents believe that he is not a safe driver any more, that they have an obligation to do something about that in general. And since they seem (?) to be relying on him for some level of care of you (driving you places?), it would make sense that they should explain to him that they don’t think he should drive you places anymore. But they shouldn’t only be protecting you, they should also tell him that they are concerned about his safety on the road altogether, and try to get him to either do what it takes to drive safer, or to consider giving up driving.
If your parents won’t talk to him about it, then it is going to come up between you and him, and you shouldn’t lie. I would find a quiet time (not right before you have to go somewhere), and tell him that you have something you need to talk about with them. I would say explain that Mom and Dad are concerned about his vision, and that it might impact his ability to drive safely, and that they have told you that it would be better for him not to drive you to Mass. I would tell him that while it is hard for me to judge those things, I trust my parents because of their experience, and because I know that they love both me and Grandpa. I would say that I love him and don’t want him to get hurt in an accident, and that I have to obey my parents (or if you are too old to have to obey them, then say that you think it is right for you to take their advice) and I didn’t want to lie to him.
The thing is, it is a very difficult thing to give up driving when you’ve been doing it for a long time. For a person to do this, it really helps to have the support of someone who will help them find ways to stay independent, and to maintain an active lifestyle. Again, this is why it would be best for your parents to get involved here, and I would be inclined to tell them that if they are concerned for my safety, that I think they should talk to him about it for his and other peoples’ safety as well.
