I would simply tell them that religious beliefs are a personal decision, and you don't feel comfortable discussing such a heated topic with family members, or anyone else for that matter. If they try to press the issue, just remember that you are not obligated to engage in a discussion.
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
