What you see as the evolution of morality among Christians is actually a different phenomenon. Most of Christian history was dominated and warped by the Catholic Church, which propounded purposefully incorrect teachings about God's revelation to man (the Bible) to sustain its political power. With the Reformation, Christians as a whole reclaimed the gift God gave them and resumed interpreting the Bible in the correct manner rather than as a tool for social control.I don't agree. I think one can say that Christianity evolved, but not by Darwinian or any other sort of blind evolution, rather it has evolved towards an ideal, much as each healthy Christian life is constantly evolving towards an ideal. This is teleological, not natural evolution. It is growth, a striving towards a goal, that goal being, of course perfect harmony with God.
When people look at the cruelty and barbarity of the early Christians, they often forget that these Christians lived in a cruel and barbaric world. Take the Inquisition, a common example of Christian cruelty: Christians didn't invent religious persecution, they grew up in it. From the first Christians persecuted by the Jews, to later Christians persecuted by the Romans, to even later Christians persecuted by Muslims, religious persecution was just they way things were. Christians did not first invent a new form of evil and then "grow out of it", rather, Christians came into a world in which that evil already existed and they had to grow out of it. Yes, they were wrong to do the evil even if it was common in their day, but they were imperfect human beings and could not become perfect immediately.
The same can be said for other things that Christians are chastised for , things such as witch-burning, religious wars, conquest, slavery, and forced conversion: in the days when Christians did such things, they were just doing what was typical in the day. When Christians stopped doing such things, only then was there anything remarkable about their behavior.
This incidentally, is why the humanist idea that mankind makes moral progress is so ridiculous. The idea of moral progress comes from a brief segment of human history --the West from about 300AD to about 1900AD. Nowhere else is this sort of progress seen except for short periods in isolated areas. And in fact it doesn't even hold in regards to the West, as Communism and Fascism were clearly backward steps in "moral evolution", backward steps that we have yet to recover. Even more ridiculously, most people who believe in moral evolution despise the primary factor that led to this moral progress: Christianity.
But all know that the world is badly broken and all feel the need for faith in some sort of redemption. If it isn't God's Blood Sacrifice, then it is moral evolution or science or Marxist theory or solipsism or space aliens that will come and save us from ourselves. Each person knows that he needs to be saved. Everyone has faith in something.