Was raised Presbyterian and my grandparents were the every Sunday type, but my mother and her two siblings certainly weren't. At best they were the "Went some Sundays" sort, but as part of the third generation, I became an atheist at an early age of 14 due to a combination of one, the church and its questionable teachings being questioned by my common sense; and two, my brain surgery sealing the deal on the whole deity question.
The first part is just as obvious as it sounds. We, as children, would have to go to Sunday school as the adults went and enjoyed coffee and cookies. They would always have some almost infantile arts and crafts activity to attempt to relay what they felt the Biblical message was in whatever the book and/or verse was that we were studying. It was primarily books like Genesis with the creation of everything and Exodus with the construction of the miraculous ark. These easily questionable ones are where I got started.
When my epilepsy began to consume my life, church had already taken a backseat (hallelujah!). Eventually, it was determined that I had to have a temporal-lobectomy performed in hopes of stopping my seizures. At the time I was taking 4650 mg of anti-epileptic medications (three separate meds) per day; and I had received about 20 EEGs, 15 MRIs, 10 CAT scans, 2 PET scans--1 that I had to fly to Toronto for before they got them here in the United States (other than at UCLA which was backed up for something like five years). None of which helped a damn bit; I was still have 1-3 seizures a month.
So, this led my young mind to say something along the lines of "That's it God! Here's where I draw the line; either you answer the prayers, stop my seizures and I'll believe in you, or you don't answer them, I continue to have them and we're done!"
Well needless to say, I was being withdrawn off of my medication for dye testing in the PET scan while simultaneously have a direct on brain EEG, so our floating white Anglo-Saxon grandpa of the clouds didn't really do much for me. At least in terms of medicine; in terms of rationale, however, he helped me greatly in becoming an atheist.