But who prays for Satan? Who, in eighteen centuries, has had the common humanity to pray for the one sinner that needed it most?

This quote is saying that, in the last eighteen centuries, no one has had the kindness or compassion to pray for Satan. People pray for everyone else, but not for Satan, who is the one sinner that needs it most. In other words, none of us are taking the time to feel compassion for the fallen one.