The excellent podcast The Rest is History has a vaguely connected series of episodes on the Roman Empire and its fall that includes a lot of discussion about the evolution of Christianity. The newest cycle, "Warlords of the West" (3 episodes on the emergence of the Frankish kingdom following the fall of the Roman Empire, a fascinating period not often talked about) and "Charlemagne" (3-part episode just on Charlemagne, who did more than any other Medieval figure ensured the spread of Christianity) have excellent about the political and religious forces that spread Christianity through Europe.