Final answer:
Christianity was recognized as the official religion of Rome by Emperor Constantine, who played a crucial role in its establishment through policies like the Edict of Milan.
Step-by-step explanation:
Christianity was recognized as the official religion of Rome in the early fourth century CE by Emperor Constantine. After his purported conversion, Constantine issued the Edict of Milan in 313, which granted religious tolerance towards Christians and stopped their persecution. Under his rule, and later during the reign of his sons, Christianity began to transition to the dominant religion within the Roman Empire. It would not become the official religion until the end of the fourth century under Emperor Theodosius, but Constantine's policies were crucial in changing the empire's religious landscape. His conversion and patronage also marked the beginning of Imperial endorsement of Christianity, which played a significant role in the religion's spread and establishment within the empire.