Religious freedom long pre-dates John Locke and his Enlightenment colleagues, as is widely thought, argues Wilken. He traces it back centuries to the writings of Christian apologists who, in the face of persecution, wrote to defend their right to practice the religion they wished without coercion. He cites the Edict of Milan, issued jointly in 312 by the Roman emperors Constantine in the west and Licinius in the east, which set forth a rationale that allowed all citizens to practice that form of piety each deemed best. Even two years earlier, he says, Galerius, co-emperor with Licinius, had...