Epicurus was an interesting ancient Greek philosopher, I find a lot of his conclusions quite sensible and moderate - that death is the end and not to be feared, that a life of moderate pleasure and doing no harm should be one's goal.
But, aside from his sensible and quite scientific ideas, he is well known for stating the following challenge to theistic arguments:
The Problem of Evil
"Is God willing to prevent evil, but not able? Then he is impotent. Is he able, but not willing? Then he is malevolent. Is he both able and willing? Whence then is evil?"
Now, some philosophers state that what we consider to be evil is the product of free will. God created the universe and man to have the greatest good, and part of that good was the ability to choose to be good or to be evil, free will.
However, while this might explain moral evils, such as genocide or murder, where human beings choose to do evil, it does not explain natural evil. Say a baby born to die of leukaemia, or a faun caught in a forest fire, caused to die in prolonged agony by its injuries. (see this PDF of 'The problem of evil and some varieties of atheism, by William L Rowe')
