Here's Tim Keller writing at the The Gospel Coalition Blog on how he discovered that even self sacrifice can be an idol.
It wasn’t until I began to search my heart with the Biblical category of idolatry that I made the horrendous discovery that all my supposed sacrifices were just a series of selfish actions. I was using people in order to forge my own self-appreciation. I was looking to my sacrificial ministry to give me the sense of “righteousness before God” that should only come from Jesus Christ. People make idols out of money, power, accomplishment, or moral excellence. They look to these things to “save them” — to give them the sense of purity, value, and acceptability that only Jesus can give. In my case, I was using ministry (and my own people) in this way.
Without the category of idolatry — a good thing turned into a pseudo-salvation — I would never have been able to see myself. Nothing but the concept of counterfeit gods could have blasted me out of my illusion of virtue and superiority. I thank God for this life-saving insight — though I still struggle mightily with the implementation of what I’ve learned.