before they essentially stole the ancient Greek pantheon?

The Romans did no such thing. For one thing, the gods that existed in the Roman Classical Period were native gods. Their names predate Greek contact, and it's simply a false, though very common, notion that the Romans simply venerated Greek deities under different names. Roman religion, though deriving from the same Indo-European tradition, was extraordinarily different from Greek religion, and though certain practices were borrowed the syncretism was never complete. Roman gods always maintained, for example, their rather detached status--there are few Roman stories of the personal activities of their gods, because even in the Classical Period it was not entirely clear that their gods, unlike the Greeks', maintained physical forms. Latin poets, it is true, spoke of their gods almost as identical in many cases, but this is a poetic convention born as much out of the Greek roots of most Latin verse forms as any religious reality. The Romans did not "steal" the Greek gods, and indeed they didn't even know about them by the time early Roman religion was beginning to take shape--much of the Greek influence in early Roman ritual is only indirect, though the Etruscans. There been plenty of work since then, but a perfectly reasonable place to begin looking to break from the wholly inadequate treatment of Roman religion in secondary education would be Dumezil's book Archaic Roman Religion. I'd also recommend Burkert's books on Greek religion in order to more fully grasp the realities of ancient ritual practice and religious conception

There are a few problems in answering this question. First off, we don't have any contemporary literary sources on early Roman religion, just writers hundreds of years later trying to come up with explanations. Second, it's very hard to define early Rome separate from Greek and Etruscan culture. The ancient world may not be as interconnected as ours, but there was still communication between Mediterranean and near eastern civilizations. Also, there was no concept of the 'canon' or sacred text like we have now, religion changed and adapted constantly, new stories were adapted and integrated, old ideas were rejected or forgotten. Greek religion was not the monolith it appears to be in Edith Hamilton, it was different in different poleis and changed over time.

So the phrasing "stole the ancient Greek pantheon" is a very modern way to think of it and not at all an accurate reflection of Roman religion. A while back, historians and anthropologists used to believe that Romans had a very different early religion based on the numina that was more animistic and less anthropomorphic. More recent work reveals that this is partly a construction perpetuated by the Romans themselves, especially in the early imperial period when they were trying to distinguish their austere, ancient religion from that of the "softer" Greeks. I believe the archaeology shows that various Mediterranean cultures were in contact as early as the 6th cent so the idea of a pure, independent Roman religion is probably not all that true.

If you're interested in some resources I would suggest starting with the Blackwell Companion to Roman Religion. I actually haven't read it myself (am a dirty Hellenist), but I've found the series is usually good for explaining fundamentals like this and would probably set you on the right track if you want to learn more.

Just to add to the two previous comments, two sources you might find helpful would be Nancy de Grummond's Etruscan Myth, Sacred History, and Legend

and

Larissa Bonfante and Judith Swaddling's Etruscan Myths