Around 1 (there is no 0 in the system), the majority of people in the Roman Empire were pagans of some sort. I imagine that the majority of Greco-Romans were Greco-Roman pagans, though others, likely among the elites, were deists/agnostics/atheists/other. According to Edward Gibbon's The Decline and Fall of the Roman Empire, Seneca stated that religion was viewed by the commoners as true, the wise as false, and the rulers as useful. Regardless of Gibbon's many historiographical issues, the point stands that not everybody believed in some sort of religious system.

Now, even among pagans, there was a huge variety of belief. Of course, there were the Greco-Roman gods, but Germanic tribes had their own pantheon. This Germanic pantheon had a war god, Tyr/Telwa, to whom captives were sacrificed after the Roman defeat in the Battle of the Teutoberg Forest (Tacitus Annals 1.61).

Beyond that, in the East, there were the Jews, as well as the assorted other pagan cults (possibly Sol Invictus in the Parthian Empire).

By the time of 100, things haven't changed all that much. The Jewish Temple in Jerusalem was destroyed (in 70) and the survivors forced to flee among the nations, creating what became known as the diaspora. Judaism, due to the destruction of the Temple, became increasingly focused on practice and tradition, rather than on sacrifice. Christianity, which came out of Judaism, was a relatively minor religion which had spread across the Empire. That's not to say that it had a huge following at all. Indeed, it seems as though Christianity didn't become the majority religion until at least the fourth century (Treadgold's History of the Byzantine State and Society).

From Christianity came a number of heretical groups (Marcionites, proto-Gnostics, etc.), but the mystery religions had also begun to develop, gaining some converts in the East. Beard's Religions of Rome is probably the best source for your question, honestly.