Hey Paul. The RG article on faith and volunteerism caused me to wonder - what about the rest of us? 
I volunteer because - just because. I donate to charity just because. I recognized that I have enough and that I can spare time, resources, and money to those less fortunate. I am not compelled by faith. My guess is that many volunteers do so as a sense of contribution for the greater good. I'm sure that a lot of 'Christians' don't do anything for anyone - outside of praying for sinners like me ;-]
I was raised Christian, as was the norm back in the day. That's another story.....