The Problem of Evil is the single most persistent issue in theology. Learn 2 of its formulations, & 1 response.
The Stanford Encyclopedia of Philosophy’s fantastic summary of many major positions on the Problem of Evil (PoE) - http://plato.stanford.edu/entries/evil/
Epicurus’s original statement of the PoE - https://s-media-cache-ak0.pinimg.com/736x/1d/ca/44/1dca444c2f4e1ed39c3d88442e19f5dd.jpg
Plantinga’s most notable work containing his Free Will Defense - http://gelogica.weebly.com/uploads/2/6/6/1/26617550/plantinga_the_nature_of_necessity.pdf
(Please note that Plantinga has provided several other arguments regarding the origins of natural evil, etc., but they have not been nearly as well-received as his refutation of the logical PoE.)
An interesting analysis of a defense first posed by C.S. Lewis in the context of other historical theodicy - https://philosophynow.org/issues/26/CS_Lewis_God_and_the_Problem_of_Evil