Set during the reign of Constantine the Great, as Christianity is introduced to Rome, these feature Claudia, agent of the Empress Helena. There are four books in the series to date (despite what Wikipedia thinks, Domina (2002) is not part of it) with hopefully more to come.
I would highly recommend reading this series in order.
In which we meet Claudia and her supporting cast, from the Emperor of Rome to the local tavern community. When courtesans that have been with the Emperor Constantine are found murdered, branded with the symbol of the newly-legal Christian religion, Claudia has to find the murderer before all of Rome is threatened.
An horrific serial killer returns to stalk the streets as Claudia investigates the murder of one of the previous emperor’s cronies – stabbed in the back inside a locked room.