People around the Roman Empire believed in many different gods, goddesses and spirits, and blended the religious practices of their own local region, of Rome, and of other regions of the Empire such as Greece or Egypt. After the time of Constantine, Christianity began to develop, but the novel is set before it became widespread.

Venus was the goddess of love and beauty. The famous Venus of Arles was discovered (broken into several pieces) at the Roman Theatre of Arles.

Lemuria rituals were held on May 9, 11 & 13. In Chapter 40, Rustica’s bare feet, hand gesture, the ritual words she chants nine times, and her actions throwing black beans and clashing bronze implements, are exactly as described by the Roman writer Ovid.
Sculptures of body parts were offered to gods and goddesses at springs to ask for healing from illness or give thanks for a cure. The three women seated in a row in the stone carving in Chapter 27 are the Celtic Mother Goddesses, who were worshipped in southern France, especially at springs.
The sacred spring described in chapters 27-28 is based on a real Roman temple at a spring in a place called Collias, not far from Avennio (Avignon), which had altars to different gods and to the Mother Goddesses.

Every Roman house had one or more small shrines, usually with statuettes of gods, goddesses or guardian spirits, to whom offerings of wine, incense, fruit or flowers were made.

Romans prayed with their hands held up to heaven at around shoulder height, and turned in a circle after completing a prayer.

Petronia and Carotus use the names of Juno and Jupiter, the queen and king of the gods, as exclamations. They would have pronounced them as “Yuno” and “Yupiter”!
Romans believed in many sorts of fortune telling. One way was to throw dice three times, to obtain a sequence of three digits. These were then matched with a line from the Greek poet Homer, listed on a special fortune-telling scroll. You can try it by using this link to an online simulator. You can see a photo of the remains of one of these scrolls here.
The Roman writer Tertullian mentions fortune telling using goats, but nobody knows how they did it! We imagined the scene in Chapter 47 by using a late Roman story of a dog who could tell whether women were pregnant and whether they would have a baby boy or a girl.