Final answer:
1) Constantine the Great was the first Christian emperor who relocated the Roman Empire's capital to Byzantium and renamed it Constantinople, now Istanbul. His endorsement and expansion of Christianity marked the start of a new Christian governance in Rome, laying the foundations of what would become the Byzantine Empire.
Step-by-step explanation:
The first Christian emperor who founded New Rome in Byzantium was 1) Constantine the Great. In the early fourth century, specifically in 330 CE, Constantine moved the capital of the Roman Empire from Rome to the ancient Greek city of Byzantion, which he later renamed Constantinople.
This city is known today as Istanbul. Following his pivotal conversion to Christianity, Constantine declared the religion legal across the empire, actively supported Christian institutions, and played a key role in the expansion of Christianity, ushering in a new era of Christian governance within the empire.
Aside from establishing Constantinople as the new political center of the Roman Empire, Constantine left an enduring legacy by being both the secular ruler and a patron of Christianity, a model that emperors like Justinian would later emulate. The city that he founded would become a significant cultural, political, and religious center for nearly a thousand years, and the nucleus of the Byzantine Empire until its fall in 1453 CE to the Ottoman Empire.