The True Cross is the cross upon which Jesus was said to have been crucified, particularly as an object of religious veneration. There are no early accounts that the apostles or early Christians preserved the physical cross themselves, although protective use of the sign of the cross was common by at least the 2nd century. Post-Nicene historians such as Socrates of Constantinople relate that Helena, the mother of the Roman emperor Constantine I, travelled to the Holy Land in the years 326328, founding churches and establishing relief agencies for the poor. The late 4th-century historians Gelasius of Caesarea and Tyrannius Rufinus claimed that while there she discovered the hiding place of three crosses that were believed to have been used at the crucifixion of Jesus and the two thieves, St. Dismas and Gestas, executed with him. To one cross was affixed the titulus bearing Jesus's name, but according to Rufinus Helena was not sure until a miracle revealed that this was the True Cross.Many churches possess fragmentary remains that are by tradition alleged to be those of this True Cross. The acceptance of these relics is generally restricted to the Catholic Church, Eastern Orthodox Church, Oriental Orthodox Church, and the Church of the East, while Protestants and other Christians dispute their authenticity. The medieval legends that developed concerning the provenance of the True Cross differ between Catholic and Orthodox tradition, as do the exact dates of the religious festivals celebrating its rediscovery. A supposed fragment of True Cross held by Waterford Cathedral was tested by Oxford University radiocarbon experts in 2016 and found to date from the 11th century. Forging of relics for sale or to promote religious tourism (pilgrimage) was common during the medieval period but so was the creation of third-class relics by touching mundane items to those believed to be holy, in the belief some of its spiritual power would be transferred by the process. Over time, many such third-class relics came to be taken as the original articles they had once touched.

Heraclius (Greek: Ἡράκλειος Herakleios; c. 575 – 11 February 641), sometimes called Heraclius I, was the Byzantine emperor from 610 to 641. His rise to power began in 608, when he and his father, Heraclius the Elder, the exarch of Africa, led a revolt against the unpopular usurper Phocas.

Heraclius's reign was marked by several military campaigns. The year Heraclius came to power, the empire was threatened on multiple frontiers. Heraclius immediately took charge of the Byzantine–Sasanian War of 602–628. The first battles of the campaign ended in defeat for the Byzantines; the Persian army fought their way to the Bosphorus but Constantinople was protected by impenetrable walls and a strong navy, and Heraclius was able to avoid total defeat. Soon after, he initiated reforms to rebuild and strengthen the military. Heraclius drove the Persians out of Asia Minor and pushed deep into their territory, defeating them decisively in 627 at the Battle of Nineveh. The Persian king Khosrow II was overthrown and executed by his son Kavad II, who soon sued for a peace treaty, agreeing to withdraw from all occupied territory. This way peaceful relations were restored to the two deeply strained empires.

However, Heraclius soon lost many of his newly regained lands to the Rashidun Caliphate. Emerging from the Arabian Peninsula, the Muslims quickly conquered the Sasanian Empire. In 636, the Muslims marched into Roman Syria, defeating Heraclius's brother Theodore. Within a short period of time, the Arabs conquered Mesopotamia, Armenia and Egypt. Heraclius responded with reforms which allowed his successors to combat the Arabs and avoid total destruction.

Heraclius entered diplomatic relations with the Croats and Serbs in the Balkans. He tried to repair the schism in the Christian church in regard to the Monophysites, by promoting a compromise doctrine called Monothelitism. The Church of the East (commonly called Nestorian) was also involved in the process. Eventually this project of unity was rejected by all sides of the dispute.