What separates a good biography from an excellent one, I think, is the author's ability to explicate the problems encountered by the subject. Only in this way is it possible to appreciate the subject's reactions and responses to them.
Stephen Williams is such an author. In his excellent and highly recommended Diocletian and the Roman Recovery he does a superb job of explaining the myriad issues facing the Roman Empire at the time of Diocletian's accession to the throne - military defense and rampant inflation to name just two.
Williams's discussion of Roman religion, the rise of Christianity during the Third Century and the problems it presented to traditionalists such as Diocletian is as fine as any I have read. Although Williams carefully lays out the many challenges that Christianity presented, I was particularly struck by his use of a "modern parable" to illustrate the "remorseless argument" that ultimately led Diocletian to sign off on the Great Persecution of Christians beginning in February 303:
A small state, brave and resourceful, is permanently surrounded by powerful enemies who threaten to destroy it. By great efforts it had success in repelling them again and again. But its government soberly realizes that, in the long run, it can only be sure of surviving if it retains the friendship (and ultimate protection) of a certain Superpower. Should this be forfeit, no amount of bravery can guarantee it against being eventually engulfed. But in this state is a noisy radical minority violently opposed to the Superpower, whose activities threaten the vital relationship. The government tries to persuade them to keep their views to themselves and show at least outward respect for the Superpower, for the sake of their country's safety. But the radicals utterly refuse such a compromise, and their movement is growing in numbers. Finally, the government's supporters urge that it has no option but to suppress this movement before irreparable damage is done.
"In this parable," Williams concludes, "the small state is Rome, the Superpower is Jupiter and the gods, and the radical minority, the Christians. It was this remorseless argument . . . that shifted Diocletian."