Final answer:
The official religion of the Roman Empire from the 300s CE was Christianity, formalized by Emperor Theodosius I in 380 CE. This evolution marked a significant transformation from Rome's traditional polytheism to a state-endorsed monotheistic religion, which greatly influenced the empire's culture and governance.
Step-by-step explanation:
The official religion of the Roman Empire starting in the 300s was Christianity. This was established by the Roman Emperor Theodosius I in 380 CE, making the empire officially a Christian state. Prior to this, Rome was known for its religious diversity and polytheism, including the worship of Roman gods and various mystery religions. Emperor Constantine's conversion to Christianity in the early 4th century was a pivotal moment, as he made it legally acceptable to be a Christian, setting the stage for Theodosius I's later declaration.
Rome's shift from polytheism to Christianity occurred alongside changes within the empire, which included riots and conflicts over religious practices, and a decline in the traditional pagan worship. The rise of Christianity marked a significant transformation in the cultural and religious landscape of the Roman Empire, and by the end of the 4th century, Christianity was not only the official religion but also played a key role in shaping the empire's policies and ideologies.