Ancient culture was incomprehensibly different from our own. Here are a couple surprising ways the ancient's basic ideas about religion were not what you'd expect.

Polytheism was one religion. There wasn't a Jupiter religion and a Mithras religion and a Serapis religion. You could—and people did—pray to Serapis or Mithras in the Temple of Jupiter.

Doctrine didn't matter. Ancient religions were not institutionalized. Even the big national religions didn't have governing authorities to standardize theologies or oversee priesthoods and practices. Doctrine didn't matter. You could ask two priests about a God, and get two contradictory versions of a God's myth and rites—at the same temple, on the same visit!

What that meant was, fluidity. Across the culture, from Spain and Britain to Egypt and the Galilee and on to India, there were hundreds of local versions of each God's faith. One God 's myth and ritual flowed into and mixed with the next's. No one cared. They saw it as all one religion anyway .

Good Books for this section

Myth and Mystery : An Introduction to Pagan Religions of the Biblical World
by Jack Finegan

An easy to read survey of pre-Christian Western religion by a mainstream scholar. Chapters on: Mesopotamia, Egypt, Zoroastrianism, the Canaanites, Greece, Rome, the Gnostics, Mandaeanism, and Manichaeism.

The power of this book is that it isn't aimed at proving a connection between paganism and Judeo-Christianity—so you're sure the author isn't skewing things to fit that argument. Yet you'll read about flood and creation myths paralleling Noah and Adam, about pre-Christian ideas of the immortality of the soul and life after death, and about lots and lots of Gods who die and are reborn.

Greek Religion
by Walter Burkert

Here's a surprise, a book by a world renown expert that's well organized and easy to read.
This book is organized by feature- of- religion: ritual, the Gods, Heroes, the dead, polytheism, the mysteries, and philosophy-religions. That gives you a compare and contrast look at, for e.g. baptism or, blood sacrifice across the culture. So the book complements the cult by cult organization of Finegan and Turcan.

The Cults of the Roman Empire
by Robert Turcan

Like Finegan's book the power of this book is that it isn't aimed at proving a connection between paganism and Judeo-Christianity—so you're sure the author isn't skewing things to fit that argument.
This book is more detailed than Finegan's—giving in depth details of the political history of the main ancient religions, and intricate details about the theology and ritual I've never seen anywhere else. Highly recommended.