I've been thinking about those symbols at the house Iusta enters when Phineas follows her - the mother and child, the fish, and was the third one a boat ?
In Ch 15 Elgin says "The symbols sound like they could be associated with Isis" and Simon adds..."The only details we know of the Rites of Isis are from Apulius and those are quite tantalising"
Isis was the patron saint of woman, magic, agriculture and much else besides. Scholars have also made comparisons between Isis worship in Roman times and the cult of the Blessed Virgin Mary.
Will Durant (History of Civilisation) wrote that :
"Early Christians sometimes worshipped before the statues of Isis suckling the infant Horus, seeing in them another form of the ancient and noble myth by which woman (i.e) the female principle), creating all things, becomes at last the Mother of God."
So it's quite possible that Iusta is a Christian bearing in mind that she was very taken with the statuette Phineas gave her of Isis and Horus - perhaps she equated them with Mary and Jesus.
I think the Tetraktys member in the group is Lyros simply because of the five year gap in his CV - though when I think about it that just seems to be too pat a coincidence.
To me the real fifth columnist is Ely - though he is not physically present (unless that really was him on the sailboat), he has been ever present throughout the story so far by way of being continually in Sophie's thoughts.
I find Maria is a bit of a worry too! She's an archeologist but all she seems to do is watch proceedings.
Love the way we get the content of Sophie's meals especially while she's recuperating from the pneumonia - tomatoes et al. There's been a lot of interest in the health benefits of tomatoes lately and while we have them regularly I must say that I seem to be eating a lot more of them while reading this book - taking a leaf out of Sophie's book (so to speak). I keep buying these perfectly ripe Baby Roma variety -just a mouthful in each but when I bite into them the flavour explodes in the mouth - I'm not so keen on the mozzarella but they go well with the nutty flavour of a good Swiss cheese - well, any cheese for that matter . A few crackers and a glass of something - Yum!