I finished the book Il Tempio della Luce by Daniela Piazza, written in Italian and published in 2012. It's a historical novel set in 15th century Milan about a group of priests overseeing a period of the cathedral's construction and raising a young boy who was the secret heir of the late Duke of Milan. The author weaves fictional characters and real people into a semi-fictional account of an assassination that actually took place.
I really enjoyed the historical descriptions because I know a lot about Milan. I found the plot to contain a lot of cruelty and violence, which unfortunately is not wrong for the time period. In the 15th century, women and children were not seen as people. Orphans were left in the middle of the streets to become beggars, and young girls were married off or sent to convents or brothels. Rulers really were ruthless (if you don't believe me, click here) and the Inquisition is a blight on the history of the church.
In this book, the priests are devout followers of a Celtic goddess and they recognize the Virgin Mary as an incarnation of her. They are trying to see the cathedral through to completion in order to bring Man back into contact with God, but what they've got in mind is not merely spiritual.
Look for the English translation.