Having grown up Catholic, I can tell you that there are plenty of "visitors" to church every Sunday. My dad, for example, was Baptist so when my mom, brother, and I went up to take our communion he would stay seated. Plenty of other people did the same.
The last few times I went to church with my mom while visiting for Easter or Christmas, my brother and I (my brother is one of those "spiritual, but not religious" people these days) both stayed seated with my dad.
I guess you could say my sin is that I would still drop a dollar in the collection plate, but that was partly because my mom's Catholic church hosts the local food bank and part of the money goes to buying more food.
EDIT: On second thought, if your family isn't aware of your atheism or doesn't approve of it, then I can see where the upheaval would come from.
I have a sin like that. I was forced (in order to prevent a shitstorm) to attend church this year. I initially refused to drop any money in the collection plate but then my mom forced a loonie into my hand. I saw my chance, and returned back to my room, took a quarter from my change container, and replaced it with the loonie. my mom saw that i had done this though, and demanded I return to my room to collect the loonie. I did so but when everyone was doing other things in terms of preparation, I sat in the porch, and reclaimed the quarter which was on the windowsill. When we went to church, I sat next to my sister, and when the time for collection came upon us, my sister asked what I had, and I only presented the quarter. she then gave me three further quarters to total a dollar, and I dropped it in, half triumphant of the fact that I gained seventy-five cents out of my only lost quarter.