If the theory of evolution by natural selection is correct, I don’t see how there can be a God who is both good and all-powerful.
Consider the question of why there is suffering in the world, why there is disease, why there is death. If there is a God, why is the world like this? Does he cause suffering, or does he just allow it; and, if he allows it, why does he do so? Does he lack the power to prevent it? Or does he allow it for some other reason?
The best answer that Christianity can give is as follows: God doesn’t cause suffering; humans do. God allows suffering it because it’s an unavoidable consequence of giving us free will. That’s essentially what the Genesis story of Adam and Eve says (regardless of whether you read it as history or as myth)—God created the world perfect, but evil, suffering and death came about by human free choice.
That was a good enough explanation until the modern era. But we now understand that suffering is woven into the very fabric of life. Natural selection only works because there is death and disease. We wouldn’t be here without death; our species would not exist. It’s only when individuals with less favourable characteristics die with fewer offspring than those with more favourable ones, that life advances. Like it or not, suffering, disease and death are part of what has created us. There was no perfect initial state; life on earth has always known death, long before humans made their appearance. And the first humans were no exception; they were part of a branch of animal life that was becoming increasingly self-aware, which meant becoming increasingly aware of pain, suffering and death. If we accept the current interpretation of the evidence in the rocks and in our own DNA, we can no longer hold onto the Genesis idea that God created a world that was perfect until humans spoiled it.
So it’s no longer enough to explain why God allows suffering as the consequence of free will; believers now have to explain why God would choose to create a world in which the development of life is founded on death in the first place. They have to explain why God would create life in this way, knowing in advance that this process would result in conscious suffering in the higher forms of life, regardless of the choices they make. To me, this stretches the idea of a good Creator beyond breaking point. I can imagine a world like ours being made by a higher being who was indifferent to suffering, but not by one who is supposed to care. I can also imagine there being a force for good that we can draw on (which we could call ‘God’ if we want to), but which can’t have been our creator. What I can’t imagine is a being who is both the source of good and the creator of a world founded on suffering.
Of course, you can choose not to believe in evolution, and believe in the Creationist version of Christianity instead, which takes Genesis literally, with its perfect original state of creation. On one level, I have some respect for that position; it may be crazy, and fly in the face of the scientific evidence, but at least it is self-consistent!—unlike the average Christian who admits that evolution must be right, and yet doesn’t think through what that implies about God.
I find it interesting that Jesus didn’t say much about the original cause of suffering in the world. People in his society tended to believe that specific cases of suffering were caused by specific sins. But when he was asked whose sin he thought had caused a man to be born blind, he made a point of contradicting the question—it wasn’t caused by anyone’s sin; instead he took it as an opportunity to do some good. And in general when he encountered people with disabilities or diseases, his main concern was to restore them into a society whose religion had made them outcasts. He didn’t use it as an opportunity to blame anyone (apart from the religious leaders who maintained the rules of exclusion). It seems that he just accepted that that’s how the world was, and got on with trying to improve it where he could. I’m all for that.
 I’m using the term ‘theory’ in the scientific sense of a hypothesis that is generally accepted as correct, because it is the best fit with all known evidence, not in the popular sense of something that’s ‘just a theory’.
 Why God is supposed to have given us free will is a whole topic in its own right; I may come back to that another time.
 Although as it happens, I don’t see any particular evidence for such a force for good, apart from our own human spirit.