St. Constantine famously had a vision where God instructed him to conquer in the name of Christ. In some respect, all stories of war-waging saints trace their origins back to St. Constantine. Prior to St. Constantine, the Christian attitude towards war was universally negative. In the centuries that followed, Church and State intertwined and this mixing inevitably has led to the use of Christian imagery in the justification of warfare.
It is often thought St. Constantine was the first Christian emperor, but this may not be the case. The first was likely Emperor Philip the Arab. Philip was notable for being sympathetic toward Christians and for winning peace with the Sassanids. Reportedly, Philip famously was excommunicated by St. Babylas during Easter, when Philip attempted to celebrate with the other Christians. During that time, there was still a general sentiment that Christians post-baptism could not join the military or hold public office, for such things were against Christian teaching. Thus, St. Babylas forced Philip to stand with the penitent Christians, who had committed some post-baptism sin.
It may be for this reason that St. Constantine delayed baptism until his deathbed, avoiding the question of holding office as a Christian altogether. In this way, he was able to live as a believer without following the same high ethical standards that might prevent him from being emperor. Despite his deathbed baptism, many believe that St. Constantine became a Christian after a vision before the battle of Milvian Bridge in 312. This battle was decisive in St. Constantine's consolidation of power within the Roman Empire.
As the story goes, before the battle, St. Constantine saw an image of the cross in the sky (sometimes it's the Chi Rho he saw), and accompanying this image was the text "In this sign conquer." Supposedly, the saint then painted the symbol on the shields of his troops and then this allowed them to win the battle.
There is much that is doubtful about this account.
The earliest accounts disagree about this vision. Lactantius's historically dubious On the Deaths of the Persecutors does not report a vision before the battle. Instead it reports that St. Constantine had a dream that he should paint crosses on the shields of his soldiers. Lactantius reports that the symbol was the tau-rho, also known as a staurogram. However, there is no historical evidence that St. Constantine used this symbol. The other early account, written shortly after the Battle of Milvian Bridge, is Eusebius's Ecclesiastical History. In this text, Eusebius speculates that God assisted St. Constantine in this battle (it was commonly believed that God would determine the outcomes of battles), but neither a vision nor a dream is mentioned.
In Eusebius's later Life of Constantine, written decades after the Battle of Milvian Bridge, we find the first description of the sign in the sky, together with the text "In this sign conquer." In this account, St. Constantine is uncertain as to the meaning of this vision, and that night has a dream which clarifies that he should use the Chi-Rho symbol as an emblem. Notably, this text does not connect this vision with the Battle of Milvian Bridge. Rather, the vision happens at some unspecified time. Thus, the account that St. Constantine saw this image before the battle is a clear confabulation of multiple inconsistent accounts, casting doubt on its historicity.
While St. Constantine did end up using the Chi-Rho as an emblem, evidence of this does not appear until several years after the battle in question, and historians interpret the material evidence as inconsistent with the story of his vision. What's more, one should interpret these tales of visions from God in light of another incident from the saint's life. In 310 it was reported that at some point, the god Apollo and the goddess Victoria appeared to St. Constantine and offered him three laurels, representing 30 years of prosperous rule. From a Christian perspective, these gods do not exist and thus this vision is either entirely fabricated, or St. Constantine was prone to false visions. This history casts even further doubt upon the emperor's later purported vision.
Of course, none of this is why the Church venerates St. Constantine. Instead, he is venerated for his Edict of Milan, and for the convening of the First Ecumenical Council. In this respect, we should interpret St. Constantine as a peacemaker, not in political affairs, but in religious ones.
St. Constantine had a vision of a great synthesis between Christianity and Greek paganism, forming a new universal civilization. The emperor attempted to enact this meeting between cultures and religions by founding churches that were dedicated to aspects of God that appealed to pagan Neoplatonists: Holy Wisdom and Holy Power. Additionally, Constantine founded a church that was to serve as the chief cathedral of the empire: Holy Peace.
At the time, however, Arianism was raging through the empire and there was much animosity between orthodox Christians and Arians. This particularly bothered Constantine, whose plans for a great union of Christian and Greek were hindered by divisions among Christians. His opinion was that some compromise should be won that allowed for a union between Arians and Orthodox Christians. He did not seem to care under what conditions that happened, so long as peace was won. To this end, he convened the Council of Nicaea. The council condemned Arius, whom St. Constantine disliked as particularly divisive. St. Constantine preferred moderates, and even disliked some of the more zealous defenders of Nicaea, exiling the great St. Athanasius. While disliking Arius himself initially, St. Constantine did come to regard Arianism as an acceptable position for citizens of his Empire, so long as it was not promoted divisively. On his deathbed, his last act was once again an attempt to create unity, having an Arian bishop baptize him. As St. Constantine purportedly said, “Division in the Church is worse than war.”
Despite all of this, we should emphasize that the Church maintained its independence from St. Constantine's vision of religious unity. The Church also, at least initially, sided against the emperor's warring tendencies. The canonical tradition of the Church for centuries after St. Constantine forbade Christian soldiers from communing, just as Emperor Philip the Arab was barred from communion. This attitude is perhaps best expressed in a letter of St. Paulinus of Nola to a soldier, written sometime after the death of St. Constantine,
Therefore do not any longer love this world or its military service, for Scripture's authority attests that "whoever is a friend of his world is an enemy of God." He who is a soldier with the sword is the servant of death, and when he sheds his own blood or that of another, this is the reward for his service. He will be regarded as guilty of death either because of his own death or because of his sin, because a soldier in war, fighting not so much for himself as for another, is either conquered and killed, or conquers and wins a pretext for death-for he cannot be a victor unless he first sheds blood. So the Lord says: "You cannot serve two masters," the one God and mammon, that is, Christ and Ceasar, even though Caesar himself is now keen to be Christ's servant so that he may deserve kingship over a few peoples. For it is not some earthly king who reigns over the whole world, but Christ God, for "all things were made by Him and without Him was made nothing. He is King of kings and Lord of lords. Whatever He pleases He does in earth, in the sea, in the deeps."