When did rome move from polytheism to monotheism?

It was when the emperor Constantine converted to Christianity. He had actually been a pagan before. In a battle, he saw in the sky the Greek symbol for Christ. Then, he heard Christ say, "In My name, conquer." Right after that, he told all his soldiers to paint the symbol on their shields, and they won the battle. Constantine became a Christian, declared it the official religion of the Roman empire, and called for churches to be built all over the empire.