God, Freedom, and Evil

While this simple set of propositions might appear inconsequential, this was one the major problems in philosophy of religion for some time. And until Plantinga, this (often called "the logical problem of evil") was one of the main arguments used against the coherence of traditional theism. Plantinga's main argument details with rigor why this set is not a contradiction (e.g., free will, metaphysics of modality, etc).
Because of this book (or the longer, more academic version "The Nature of Necessity") philosophers have all but stopped discussing the logical problem of evil. This rarely rarely happens in philosophy which is a testament to Plantinga and his book. Buy it, read it and spread the word. It is not logically inconsistent to believe that a good and all-knowing God would allow evil.