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.
A meeting of the factions of the Christian church is disrupted first when a priceless relic vanishes from a locked room and then when one of the delegates is found staked out on the grass with hundreds of cuts to his body. And Claudia is busy proving Murranus innocent of poisoning a rival.
Children are being kidnapped from the richest families of Rome and veteran soldiers from a campaign against the Picts are being executed in a traditional Pict manner – namely their bodies being found with a part of the male anatomy removed and placed in their hand.