Should I doubt my religion even though I know it will crush my extremely caring grandfather who raised me Christian?
That depends. What are you scared of losing?
I don't wanna lose my grandfather after he died
It is scary to be honest with people especially if you could lose them but it is not good to lie to the people you care about. They would feel betrayed that you lie to them.
I know it's scary but I'm afraid the reaction from the rest of my family members.
Even though it is hard to not cling on to how you were raised by your grandfather, you have to be true to yourself. And you should be honest to the people who you care about. You shouldn't hide such a big part of yourself from them, otherwise they will feel betrayed.