In the same way that earlier Roman emperors had claimed a special relationship with a deity to augment their authority and standing (Caracalla with Serapis, for example, or Diocletian with Jupiter ), Constantine chose the figure of Jesus Christ. At the First Council of Nicea (325 CE), he presided over the gathering to codify the faith and decide on important issues such as the divinity of Jesus and which manuscripts would be collected to form the book known today as The Bible . He stabilized the empire, revalued the currency, and reformed the military, as well as founding the city he called New Rome on the site of the former city of Byzantium (modern day Istanbul) which came to be known as Constantinople.
The Romans, however, managed to stop further Islamic expansion into their lands during the 8th century and, beginning in the 9th century, reclaimed parts of the conquered lands.  In 1000 AD, the Eastern Empire was at its height: Basil II reconquered Bulgaria and Armenia, culture and trade flourished.  However, soon after, the expansion was abruptly stopped in 1071 with the Byzantine defeat in the Battle of Manzikert . The aftermath of this important battle sent the empire into a protracted period of decline. Two decades of internal strife and Turkic invasions ultimately paved the way for Emperor Alexios I Komnenos to send a call for help to the Western European kingdoms in 1095.