Who was the first Roman Emperor to greatly promote Christianity in Ancient Rome?

Emperor Constantine I (also widely known as Constantine the Great or Saint Constantine) is greatly credited with making Christianity the official religion of the empire (although some people credit Emperor Theodosius, who came to power 42 years after Saint Constantine's death and passed multiple laws against paganism).

Constantine I (February 272 AD – May 337 AD), was Roman Emperor from 306 AD to the moment of his death. He became one of the best known rulers and religious figures of his epoch, also being a significant figure in the history of Christianity. The holiest place in Christendom, the Church of the Holy Sepulchre, was built and consecrated on Constantine's orders in 335.

Constantine died at the approximate age of 57. As he is thought to have been baptized on his death-bed, he is called the first Christian Emperor of Rome.