Heraclius

In 610 CE, Heraclius changed the official language of the Empire from Latin to Greek.
And in 629 CE, he changed the Title of the Roman Emperor from ‘Augustus’ or ‘Imperator’ to the Greek ‘Basileus’.

By 611 CE, the Persians had conquered Mesopotamia and the Caucasus. They occupied Syria and had reached Chalcedon opposite Constantinople, after pillaging Asia Minor.
By 618 CE, the Persians had captured Jerusalem, the Levant and Egypt.
Heraclius contemplated moving the capital from Constantinople to Carthage in North Africa, as they were no longer receiving Grain from Egypt, and was forced to buy off the Persians with Tribute.
Meanwhile the Balkans were invaded by the Avars and Slavs, and the Persians were starting to occupy islands in the Aegean, including Rhodes.

The Siege of Constantinople (626 CE)

The Eastern Roman Empire was faced with near extinction.
An army of Avars, Slavs and Sassanid Persians besieged the Walls of Constantinople and two Fleets lay in the Bosphorus.
An attempt at invasion by sea was prevented and then a storm wrecked their Fleet. The Avars and Slavs withdrew, and the Persian army at Chalcedon became irrelevant.
In 627 CE, Heraclius defeated a Persian Army at the Battle of Nineveh in Mesopotamia
After this the Persians signed a Peace Treaty, ending the Roman Wars with Persia, which had begun in 54 BCE. Civil wars then kept Persia in a state of decline until the Arab Conquest.

The True Cross was considered by the Western Catholic Church and the Eastern Orthodox Church to be the original Cross of Christ’s Crucifixion. It was thought to have been placed in Jerusalem by Helena in the 330’s CE.
In 614 CE, the Persian King Khosrau II removed it from the Church of the Holy Sepulchre in Jerusalem, and took it back to Persia.
After defeating Khosrau II In 628 CE, the Heraclius regained possession of the True Cross and returned it to Jerusalem in 630 CE.

Between 633-654 CE, the Rashidun Caliphate was engaged in a twenty-one year war against the Sassanid Empire which finally collapsed and resulted in the conquest of Persia.
Between 634-656 CE, the Rashidun Caliphate fought a twenty-two year war against the Byzantine Empire, which eventually lost all of its eastern Provinces.
The Rashidun Army started with the conquest of the Levant (634-638 CE), then the conquest of Asia Minor (637-656 CE), the conquest of Egypt (639-641 CE), the conquest of the Exarchate of Africa which became a Roman Client Kingdom paying Tribute (641 CE), the campaigns against Nubia, which became a trading partner of Egypt (642 and 652 CE), the conquest of Sicily (652-654 CE) and the conquest of Cyprus with 500 ships (654 CE).
After the Battle of the Masts fought near Antalya in 655 CE, where a Roman Fleet lost 500 ships to an Arab Fleet, the Naval domination of the Mediterranean by the Roman Empire ended.
This led to the First Arab Siege of Constantinople between 674-678 CE.