I don't know how devout your father is, but in my opinion if you tell him that while you personally just cannot believe in God / Jesus as beings that exist that you can understand why he does and admires the steps he has taken to become a better person. Maybe add that while you are secular, state that there are some good morals to be found in the Bible and will follow them (can't think of any off the top of my head).

Of course that could be all just a massive lie and you think it's all horseshit to prey on the ignorant, but it may make your life a hell of a lot easier while you live with him. Just don't be antagonistic and if he's a prick about you not believing in the sky man then you were pretty fucked from the start and just hold out until you become an adult.