The existence of evil is one of the most powerful arguments against theism (and particularly Christianity): how can an all-powerful, all-loving God permit evil in His Creation?
I think that there are two important responses to make to this. First, that Christianity can answer this problem. Second, this argument requires the existence of God, because it starts from the self-evident fact that objective good and evil exist. Without God, you can’t have objective morality; and without objective morality, you can’t claim that this world has evil.
I address this (mostly the second point) in greater depth in a post that Strange Notions has reposted today. Check it out!