No issue has caused me to question my Christian beliefs more than what is often called the problem of evil.  Ever since the Greek philosopher Epicurus (342-271 BCE) first questioned the existence of God due to the existence of evil in the world, people have faced this problem and often walked away from the faith of their childhood.  The reality of suffering in light of the professed goodness and power of God is the catalyst that is most responsible for the shift in my own theological views.