My lesson is on "The Barbarians and the Empire," part of my Sunday School class on Church History (The First Thousand Years). There's not a lot of theological profundity in this particular presentation, but I've got to disentangle 150 years of the Völkerwanderung that tore the Roman Empire apart and gave birth to modern Europe. It's the critical boundary between the Classical and Medieval worlds, and it is usually ignored by history teachers. But it explains so much.
Besides, it's fun.