I feel sorry for you man. When I told my parents I was an atheist when I was 15, they simply stopped forcing me to go to church and that was it. No attempts whatsoever to get me to reconsider my position, no insults about how evil I was, they simply realized it was my choice and that they still loved me no matter what I felt about god.
So based on my experience I would try to explain to them that their beliefs are just beliefs, and not everyone believes the same thing. Then explain that a difference in beliefs shouldn't make you any less of a family, but if it did, the family was lacking a strong enough bond before any of these issues arose. Remind them you are still family, and beliefs can't tear apart a strong family on their own.
So based on my experience I would try to explain to them that their beliefs are just beliefs, and not everyone believes the same thing. Then explain that a difference in beliefs shouldn't make you any less of a family, but if it did, the family was lacking a strong enough bond before any of these issues arose. Remind them you are still family, and beliefs can't tear apart a strong family on their own.
Even if the open windows of science at first make us shiver after the cozy indoor warmth of traditional humanizing myths, in the end the fresh air brings vigor, and the great spaces have a splendor of their own - Bertrand Russell
