[Disclaimer: I got this book via LibraryThing's Early Reviewer program]
The first thing to know about this book is that the title is misleading. True, the text explains why it says "phoenix", but the "fields of Mars" are not related to the Red Planet but just to the battlefields.
The book starts in the present days' Jerusalem (with some explanations which are a bit patronising for an European - this is the weakest part of the book), but Kyla Marshall, the main character got pushed back to 8 A.D. together with four soldiers. No, Jesus Christ is not directly related to the story: the soldiers must stop a rogue scientist who went back in time to spread a deadly virus among the Germans - does anybody remember the sentence "Quintili Vare, legiones redde!" spoken by Augustus? The story runs well, but I could not stand that the plot comes to Venice. True, Regio Venetia then existed: but it took four centuries for the city to be established...