I have read _about_ these books but not actually read them. Both deal with the post-Theodosius conversion of Germanic and Scandinavian peoples and appear to focus on the use of Christianity as a political tool of rulers. The MacMullen book in particular is very hostile to Christianity. Fletcher is more balanced.
I’d recommend Charles Norris Cochrane’s book “Christianity and Classical Culture,” published by Liberty Fund, for a classic treatment of the process of conversion in the early centuries. “Pagans and Christians” by Robin Lane Fox is also highly regarded. Both of these focus on Rome.