The most common theodicy is the free will theodicy. This is that God created evil so that we could then choose between good and evil, and make moral choices. If all choices result in good, there would be no moral choices. If love is acceptable, it must be chosen over hate and therefore evil and suffering result when we make morally poor choices. However this classical theodicy does not hold up, for many reasons. Prominent historical Christian theologians who have rejected the free will theodicy include St Augustine, Martin Luther and John Calvin1. The arguments on this page are thousands of years old, but, many continue to believe in the simplicity of the free will theodicy, so, it does no harm to state the arguments against it again.
These are the menu headings on my page about the free will theodicy:
The contents of "Is Free Will the Reason God Allows Evil and Suffering?" by Vexen Crabtree (2003) is now: