This isn't so much a novel as a collection of short stories, each one worth reading in its own right.
As an alternative history it is based upon one imagined change in world history: if the Hebrews had never escaped from Egypt then Christianity would never have risen in Palestine and changed the way people think about the world, their place in it, and morality. The hypothesis seems to be that without the Christian principles of charity and social justice, the Roman Empire would never have fallen.
Credible? I don't think so, but in this book that doesn't seem to matter. There are oversights and assumptions - anyone who has studied Roman history will be disappointed by them, but they really shouldn't be. This is a very enjoyable read.
The pictures of Roman Europe (and beyond) you get from reading these stories are vivid and imaginative, but you aren't asked to stretch your imagination too far; it seems to work. It makes sense. You can believe it would have been possible. That's what makes this such an enjoyable book.