Because emperor Constantine adopted it as the national religion of the empire to try to unite it
Christianity did not easily spread through the Roman Empire. See the related question, "How did Christianity spread throughout the Roman Empire?"
As Roman Emporer, Constantine declared Christianity to be the official religion of the Roman Empire. At the time, the Empire was vast, so Christianity spread quickly.
The Roman Empire.
There was only one Roman Empire. Moreover, the Roman Empire did not study Christianity. An empire cannot study. The Roman Empire was where Christianity spread from Judea (which was a part of the Roman province of Syria). Both Catholic and Orthodox Christianity developed in the Roman Empire. Originally the were called Latin or Western Christianity and Greek or Eastern Christianity respectively. The former was the dominant form of Christianity in the western part of the Roman Empire and the latter was the dominant form of Christianity in the eastern part of this empire. There were also dissident Christian doctrines and sects.
Do you mean Christianity?
Christianity
That would be Christianity.
Roman empire.
The emperor Theodosius I declared Christianity to be the official religion of the Roman empire. In fact, he made Christianity mandatory for all.
After the Emperor Constantine converted to Christianity, that became the official religion of the Roman Empire.
Through most of their history the ancient Romans had their own religion: Roman Religion. In the Later Empire Christianity spread in the Roman Empire, was endorsed by the Roman emperors and became state religion.
The Roman Empire made Christianity the official religion of the Empire, and 'encouraged' the peoples it conquered to convert.