Hello Zeke,
Not sure about the gods, i read that lead poisining wasn't really a factor as not much has been found in the bodies of ancient romans, I dont't think (apart from the Goths) the romans actually let them in, i believe they just went without asking. Poor leadership was often a problem, and as stated elsewhere logistics was always a problem too....
The impact of christianity however was huge, not only on Rome but the whole world. The roman pantheons were dwindling in importance long before christianity, and many foreign cults and religions were happyily absorbed by the Romans, but christianity was just the only monotheistic one i believe, while the other religions had room for more then one, chrisitanity was either or....
cheers
viggen