"Is God willing to prevent evil, but not able? Then he is not omnipotent. Is he able, but not willing? Then he is malevolent. Is he both able and willing? Then whence cometh evil? Is he neither able nor willing? Then why call him God?"- Epicurus
If God is truly omnipotent, then he could stop evil. He doesn't, so he must not be omnipotent right? If God created men with free will, then he couldn't possibly be all powerful and all knowing because he would know in advance our decisions, meaning that our actions are pre-determined (hence, there is no free will).
Think about it.