First legalization of Christianity: On April 30, 311, Galerius issued the Edict of Serdica, ending the Great Persecution in the Roman Empire. Credit: flickr / Steven Zucker CC BY-NC-SA 2.0 On April 30, 311 AD, the Roman Emperor Galerius issued what became known as the Edict of Serdica, which made the first formal move to legalize the Christian religion, ending nearly a decade of brutal persecution. Though history tends to credit Constantine, wit…