The number one objection to the existence of God, by far, is the reality of evil. But there are two points to remember when you are confronted with this question.
1) It assumes that there is actual evil in the world.
2) Both those who believe in God, and those who don't, have to be able to offer an explanation for it.
Considering 1): for those who believe in Christianity, there is an explanation that fits both what we see in the actual world and what the Bible tells us about it. As it turns out, these match up very consistently and reasonably. Watch this video and give it some thought ...
As it turns out, Christianity has a very unique outlook regarding evil and suffering because the God who allows us free will is also a God who has shared in our experience of suffering. But, at the same time, He also loves us in spite of our rebellion and offers Himself as a rescuer from its consequences.
No other God does such a thing.
Keeping these in mind, we understand that for us to claim something is evil requires that there must be some kind of standard that exists outside of us by which evil is measured. That Standard is God's character. Good is a reflection of Him. Evil is the absence of His character.
Considering 2), any worldview that denies the existence of God has no basis for claiming anything as "evil." There may be things that people don't like -- personal preferences they think are bad -- but without a standard to measure those things by, there is no way to claim they are evil.
As it turns out, atheism has no basis for saying anything is wrong, or bad, or evil beyond the fact that atheists don't like it. Christianity has a perfectly reasonable explanation for evil.
Evil ends up being proof that God exists.
Please understand that this does not mean atheists can't be moral, nice, kind people. It just means that their worldview has no explanation for why they should be moral, nice, kind people.
Why is a good God more plausible than an evil God?
Other Blog Posts
"The Cries That Bind": An essay on the common human tendency to wonder where God is when we see or experience suffering, and how asking that question puts us in the company of faithful giants.
"Biblical Glass Houses": Are we hypocritical in saying that there is a difference between the violence we see perpetrated by Israel in the Bible and the jihad we see being perpetrated by Islamic terrorists today?