Good Deeds and Christianity

In every religious discussion I've had with a Christian, one point inevitably comes up. I've encountered a few variants, but it tends to go something like this:
By criticizing religion and focusing on all the atrocities committed in the name of Christianity, you are forgetting about all the good deeds for which Christianity is responsible. You are throwing the baby out with the bathwater. Christianity has been responsible for some awful things, but we must not forget the many positive outcomes.
If we examine this common argument more closely by plugging in some specifics, the point becomes clearer. A favorite example of progressive Christians is Martin Luther King Jr. In God's Politics: Why the Right Gets It Wrong and the Left Doesn't Get It, Jim Wallis writes,
But the answer to bad theology is not secularism; it is good theology. It is not always wrong to invoke the name of God and the claims of religion in the public life of a nation, as some secularists say. Where would we be without the moral prophecy of Martin Luther King Jr., who held his Bible in one hand and his Constitution in the other as he preached, holding us to our best values? Can anyone deny the prophetic leadership of Archbishop Desmond Tutu in South Africa and Oscar Romero in El Salvador? Implicit in this argument is the suggestion that these leaders (King, Tutu, and Romero) did what they did because of their faith. Maybe their faith was a contributing factor, but does this mean that without their faith, they would not have bothered? This seems unfair to the characters of these leaders, giving religion perhaps too much credit while overlooking their qualities.
Many Christians desperately want to believe that religion/god is a necessary condition of good deeds. But a compassionate, ethical, motivated, and empowering individual is capable of doing all sorts of good deeds with or without religion. A good person is a person who consistently performs benevolent acts. A bad person is one who consistently performs malevolent acts. Most of us are not entirely consistent and fall in the large gray area between these extremes. Even a cursory examination of history reveals that religion is neither a necessary nor a sufficient condition of performing positive acts.

This is the point where the eyes of my Christian readers are lighting up.
Ah ha! I've got you now! You are trying to blame religion for the bad while saying that it has nothing to do with the good. You can't have it both ways.
But I am not claiming that religion is a necessary or sufficient condition for performing negative acts either. Jerry Falwell would not be a better person without religion. Falwell, Robertson, and the like have repeatedly demonstrated that something is seriously wrong with them. They are going to be hateful, intolerant individuals with or without their religion.

Briefly consider the case of a former atheist who becomes a Christian. I would not say that such a person was morally better when she was an atheist than she is now as a Christian. I also wouldn't say that she's any worse.