The depth of the problem of evil is frequently not appreciated by either believers or, ironically, by non-believers. If someone believes that there is a God, then they are committed to the view that every single instance of suffering that any sentient being has suffered in the entire history of the universe is such that it could not be decreased, eliminated, or altered in any way without making the world, on the whole, a worse place.
An infinitely powerful, knowledgeable, and good being would not tolerate the existence of any truly gratuitous or pointless evil. So the believer can’t be satisfied merely with the possibility that there could be a God and that that God could possibly have optimized every instance of suffering in the universe. In order to be reasonable in believing that there is such a being, the believer’s sum evidence must indicate that in fact, there has never been a single instance of gratuitous suffering or an instance of suffering that could have been reduced, eliminated or altered in any way without making the world a worse place.
The irony here is that on a daily basis, we all operate with the view that there are countless instances of suffering that should be eliminated, reduced, or altered in order to make the world a better place. We see homeless people on the street that need help, there are countless people suffering from war, disease, famine, and starvation. There are animals that need to be protected. The examples of suffering that we ordinarily take to be gratuitous are countless. Call all of those cases the evidence for gratuitous evil. To believe in God reasonably then, one needs to have such compelling evidence that there is an omni-being that it eclipses and is more convincing that the evidence for gratuitous evil. That is, one needs to have better evidence for the existence of a divine being who would not tolerate any gratuitous evil than one has evidence for the existence of any gratuitous evil.
So here’s the crux: many people have argued for the existence of God, although the consensus among philosophers of religion is that no such arguments are successful. And many people believe that they have evidence for the existence of God. But does anyone think they have evidence for God that is more compelling than the evidence that we all have for the existence of gratuitous suffering in the world?