As the knowledge of one God was imparted to all men, a single sovereign arose for the entire Roman empire and a deep peace took hold of all
Eusebius Life of Constantine
The shadow of both the emperors Constantine and Charlemagne loomed large over the pilgrimage to Compostela. 
Along the pilgrimage roads to Santiago the sculpted image of a victorious rider recurs on the facades of churches. Invariably, beneath is a cowering figure being trampled upon. Who is this mysterious horseman? Commentators have long agreed that it represents an archetypal Christian monarch triumphing over paganism but it seems impossible to distinguish which of the two most likely historical figures it might be: Constantine or Charlemagne, for both emperors developed legendary features fitting the same mythological purpose.
By the edict of Milan in 313 the Roman Emperor Constantine had granted Christianity freedom from persecution after a lengthy period of oppression at the hands of his predecessor Diocletian. Constantine was responsible for Christianity becoming the state religion of the Empire even though it is often questioned whether he was a believer himself. Nevertheless, he ordered the construction of the first church of the Holy Sepulchre at Jerusalem and Saint Peter’s in Rome. He called the Council of Nicaea in 325 at which the Nicene Creed, the expression of the Christian Trinitarian doctrine was devised.
His reputation grew in the Christian imagination during the middle ages largely because of the account of his life written by Constantine’s counselor, Eusebius bishop of Caesarea. According to this, when Constantine was planning to fight the battle of the Milvian Bridge in 312 against the army of Maxentius, his rival for control of the empire, he experienced a vision which persuaded him to order his forces to fight under a Christian banner. His authority as a Christian ruler was further revered due to the document known as the Donation of Constantine, an early medieval forgery, which granted temporal power to the Pope.
The Franks were the first Christianised Germanic tribe and their ruler, Charlemagne was seen as Constantine’s successor, acquiring a vast empire the size of which had not been seen since the decline of Rome itself. Charlemagne was a conquering Christian monarch who, although he fought mainly against pagans in the Germanic lands, was conflated in legend with his father Charles Martel who had defeated the Arabs at the battle of Poitiers in 732. This halt to the Arab march through Europe was a legendary victory, the wishful prototype of later encounters with the Saracens of the Crusader era.
Charlemagne did much to restore a declining monastic culture in Europe and the Benedictine order of the eleventh and twelfth centuries looked to Charlemagne as their secular champion. Monasteries attributed their foundation to him as a way of asserting their power and autonomy and at a time when the Papacy was threatened, Charlemagne was its protector.
On Christmas Day 800 Charlemagne revived the memory of Rome when he had himself anointed the first Holy Roman Emperor.
In referring to himself as emperor, Charlemagne may not have been thinking solely of his secular role. The notion of empire had been embedded in Apocalyptic prophecy since the time of the Book of Daniel and continued through the Book of Revelation and into the middle ages. These prophecies defined the span of earthly time in terms of four empires. They were either the Greek, Persian, Roman or Arab depending on the contemporaneous setting, but they were agreed that there would be a final evil empire which would precede the End of the World.
According to legend, Charlemagne was the first pilgrim to Compostela and liberator of the shrine of the Apostle James
Images: 1. Equestrian figure Saint-Pierre at Parthenay-le-Vieux. 2. Mosaic from Hagia Sophia. 3. Manuscript Fulda 10th century