Below is an artist's rendition of the famous Vision of Constantine. There are two versions of this vision viz: 

1] one is before the battle of Milvian Bridge, where Constantine is said to have witnessed, about midday, a 'cross' in the heaven's. This image of a cross was later, via a further dream, said to have been associated with the cross of Christ and that in fact it was Christ himself, through this dream, who had told Constantine that using the 'cross' as a 'talisman' would help him triumph against his enemies. Those in the retinue of Constantine who were already Christian took the opportunity to tell Constantine that it was 'their' God who would make him victorious!

2] The second vision was dated much earlier in his career - around 310AD which took place in Gaul . It led one scholar to say when talking of the 'hyped up' Milvian Bridge vision that: "what we might have here is a later Christian interpolation of an originally pagan account. Perhaps this interpolation was made easier by the fact that, as Chadwick suggests, "Constantine was not aware of any mutual exclusiveness between Christianity and his faith in the Unconquered Sun". [SEE HERE]. In this earlier account, on the way to besieging Massilia (Marseille), Constantine and his army stopped at the site of a Temple [sometimes identified as Grand or perhaps Autun] where he enters this temple of the Celtic God Apollo. In that temple he has a vision, seeing the great God Apollo next to Victory. 

In the famous and anonymous panegyric of 310AD written in favour of the emperor, it reports that "having turned off from the road to visit ‘the most beautiful temple in the world’, Constantine was greeted by a remarkable sight: ‘For you saw, I believe, Constantine, your Apollo, accompanied by Victory, offering you laurel crowns, which each brought an omen of thirty years [of life or rule]’. The orator continues: 'And why do i say 'I believe'? You saw and recognised yourself in the likeness of him whom the divine songs of the bards had prophesied that rule over the world was due!'.

Oxford scholar Bardill surveyed all the evidence and came to the conclusion that: "It is therefore conceivable that the earlier account of the vision [i.e. the one in Gaul] inspired the stories of the dream and the vision of a symbol bestowed by the sun.....A substitute for the Gallic vision would, it may be argued, have been created to suit Constantine's own political and religious agenda's .... Constantine and his advisors may have found it desirable to replace the original firmly pagan vision of Apollo and Victory with an imperial experience more ambiguous as to its religious import. ...the symbol's precise form might deliberately have been left vague, making it susceptible to both pagan and christian sentiments" [Constantine, Divine Emperor of the Christian Golden Age].

This symbol of this Cross and its 'precise form' might have been left deliberately vague and this vagueness and the exact nature of the 'Cross' is echoed here when one observer wrote:

"This, no doubt, will appear all very strange and very incredible to those who have read Church history, as most have done to a large extent, even amongst Protestants, through Romish spectacles; and especially to those who call to mind the famous story told of the miraculous appearance of the cross to Constantine on the day before the decisive victory at the Milvian bridge, that decided the fortunes of avowed Paganism and nominal Christianity. That story, as commonly told, if true, would certainly give a Divine sanction to the reverence for the cross. But that story, when sifted to the bottom, according to the common version of it, will be found to be based on a delusion -a delusion, however, into which so good a man as Milner has allowed himself to fall. Milner's account is as follows: "Constantine, marching from France into Italy against Maxentius, in an expedition which was likely either to exalt or to ruin him, was oppressed with anxiety. Some god he thought needful to protect him; the God of the Christians he was most inclined to respect, but he wanted some satisfactory proof of his real existence and power, and he neither understood the means of acquiring this, nor could he be content with the atheistic indifference in which so many generals and heroes since his time have acquiesced. He prayed, he implored with such vehemence and importunity, and God left him not unanswered. While he was marching with his forces in the afternoon, the trophy of the cross appeared very luminous in the heavens, brighter than the sun, with this inscription, 'Conquer by this.' He and his soldiers were astonished at the sight; but he continued pondering on the event till night. And Christ appeared to him when asleep with the same sign of the cross, and directed him to make use of the symbol as his military ensign." Such is the statement of Milner. Now, in regard to the "trophy of the cross," a few words will suffice to show that it is utterly unfounded. I do not think it necessary to dispute the fact of some miraculous sign having been given. There may, or there may not, have been on this occasion a "dignus vindice nodus," a crisis worthy of a Divine interposition. Whether, however, there was anything out of the ordinary course, I do not inquire. But this I say, on the supposition that Constantine in this matter acted in good faith, and that there actually was a miraculous appearance in the heavens, that it was not the sign of the cross that was seen, but quite a different thing, the name of Christ. That this was the case, we have at once the testimony of Lactantius, who was the tutor of Constantine's son Crispus--the earliest author who gives any account of the matter, and the indisputable evidence of the standards of Constantine themselves, as handed down to us on medals struck at the time. The testimony of Lactantius is most decisive: "Constantine was warned in a dream to make the celestial sign of God upon his soldiers' shields, and so to join battle. He did as he was bid, and with the transverse letter X circumflecting the head of it, - [ie staurogram] he marks Christ on their shields. Equipped with this sign, his army takes the sword." Now, the letter X was just the initial of the name of Christ, being equivalent in Greek to CH. If therefore, Constantine did as he was bid, when he made "the celestial sign of God" in the form of "the letter X," it was that "letter X," as the symbol of "Christ" and not the sign of the cross, which he saw in the heavens. When the Labarum, or far-famed standard of Constantine itself, properly so called, was made, we have the evidence of Ambrose, the well-known Bishop of Milan, that that standard was formed on the very principle contained in the statement of Lactantius--viz., simply to display the Redeemer's name. He calls it "Labarum, hoc est Christi sacratum nomine signum."--"The Labarum, that is, the ensign consecrated by the NAME of Christ.' There is not the slightest allusion to any cross--to anything but the simple name of Christ...☧'

This was written in 1853, over 150 years ago but it still shows the confusion over what really happened during Constantine's vision and what was shown on the Labarum. It is this confusion that Chérisey is preoccupied with. Why does he want to link it in strange ways to Bérenger Saunière and a code in some mysterious parchments?

Lactantius, writing only 3 or 4 years after the 'vision' of Constantine doesn't describe a cross. He calls what is seen a 'symbol' - which was to be understood as 'a sign of Christ' - not 'a sign of the Cross'. Some observers feel therefore that it is not a 'Cross' or staurogram or other image of a Cross that became the sign of Constantine's 'vision' but something else. This something else was the Chi-Rho! No satisfactory explanation of this change from a 'cross' to a 'chi-rho' has ever been offered [i.e. the change from a kind of staurogram to the chi-rho or ☧]. 

In “The Staurogram: Earliest Depiction of Jesus’ Crucifixion” the March/April 2013 issue of Biblical Archaeology Review, Larry Hurtado highlights an early Christian crucifixion symbol - a staurogram created out of the Greek letters tau-rho: “In Greek, the language of the early church the capital tau, or T, looks pretty much like our T. The capital rho, or R, however, is written like our P. If you superimpose the two letters, it looks something like this:.

Some have seen this Tau Cross as a pre-christian symbol of a Messianic claimant. Some even ask if King Herod was a Messianic claimant. Simcha Jacobovici is in agreement when he writes on his website [http://www.simchajtv.com/] about the work of David Wray;

"Everyone agrees that at some point the staurogram became an early symbol of Christianity. It combines the Greek letter “Tau”, which is shaped like a “T” and the Greek letter “Rho”, which is shaped like a “P”. Put them together and you've got an image that came to mean Jesus on the cross. But, recently, Professor Larry Hurtado wrote an article for Biblical Archaeology Review which pointed out pre-Christian uses of the staurogram e.g., on a coin that King Herod used in the year 37 BCE i.e., some 60 years before Jesus’ crucifixion. So the question is; was the staurogram a non-messianic symbol that Christians turned into a messianic symbol? Or was the staurogram a messianic symbol before Jesus, and Christians merely adapted it for their messiah?

In the short article below, my friend David Wray, author of the excellent “The Secret Roots of Christianity: Decoding Religious History with Symbols on Ancient Coins”, suggests that the staurogram, the cross and even the “Chi” for “Christos” i.e., the “Christ”, were on King Herod’s coin some 40 years before the birth of Jesus of Nazareth because they were the symbolic currency that a king used to claim to be the anointed one i.e., the “Christ”.

Put simply, it seems that King Herod claimed to be the messiah or “Christ” before Jesus was born and, according to the 1st century historian Josephus, King Herod’s grandson, Herod Agrippa, was hailed as a “messiah” after Jesus was crucified. It’s logical, therefore, to assume that King Herod’s son, Herod Antipas, also thought of himself as a messiah or “Christ”. This would go a long way to explain why Antipas turned Jesus over to the Romans, who sent him to the cross. It seems that the clash between Jesus and Antipas was not one between a revolutionary and the regime. Rather, what the Gospels are describing is a clash between two individuals – both claiming to be messiah.

[David Wray writes] Why was a staurogram (¬ ), a symbol that combines a tau (T) and a rho (P) representing the crucifixion of Jesus, displayed prominently on a coin minted in Samaria under King Herod the Great of Judaea? New Testament scholar, Professor Larry Hurtado, recently wrote an article about staurograms using this coin to exemplify pre-Christian, non-messianic use of the symbol. He said that King Herod used the symbol in 37 B.C.E. to represent the Greek word “trias,” meaning three, indicating the third year of King Herod’s reign. However, numismatists (experts on coins) don’t all agree with Professor Hurtado’s interpretation. You see, he ignored all the other symbols on the coin. On the “heads” side (left) a star rises above a military helmet as though at the end of a scepter. This recalls Balaam’s curse (Numbers 24:17), the original prophecy from the time of Moses about the coming of a messiah, “I shall see him, but not now: I shall behold him, but not nigh: there shall come a Star out of Jacob, and a Sceptre shall rise out of Israel, and shall smite the corners of Moab, and destroy all the children of Sheth.” Palm branches next to the star recall that palm branches, symbols of rejoicing and triumph, were strewn on the path before Jesus on Palm Sunday. No Jew in the first century B.C.E could look at these symbols without thinking of their foretold militaristic messiah. The number three appears on the “tails” side of the coin, but that’s only part of the story. Greek letters circle the edge of the coin saying “of King Herod.” An incense altar in the centre recalls incense burning in the Second Temple. To the left of the burner, the Greek letters L and Γ are a date, literally “year three” of King Herod’s reign. Instead of indicating “three,” the staurogram to the right of the burner actually combines a chi (X) and a rho (P), representing “christos,” the Greek word for “anointed.” In Hebrew, that’s moshiach (messiah)! How could this be possible? In 40 B.C.E., the Parthians and Romans each anointed a “King of the Jews” and fought a war to determine which puppet would rule Judaea. The Parthians anointed Mattathias Antigonos and the Romans anointed Herod. After three years, the Romans won and installed King Herod on his throne. As the first Jewish king to use only Greek on his coins, he combined a chi and rho on his coins to symbolize himself as the rightfully anointed (christos) King of Judaea. King Herod used similar symbols even more explicitly on a coin minted in Jerusalem.

The earliest Christian uses of this tau-rho combination make up what is known as a staurogram. In Greek the verb to 'crucify’ is stauroō; a ‘cross’ is a stauros … [these letters produce] a pictographic representation of a crucified figure hanging on a cross—used in the Greek words for ‘crucify’ and ‘cross.’ The tau-rho staurogram is one of several christograms, or monogram-like devices used by ancient Christians, to refer to Jesus. However, Larry Hurtado points out that the staurogram only refers to the crucifixion, unlike others, which mention Jesus’ other characteristics. Also, the staurogram is visual—the tau-rho combinations create images of Jesus on the cross, making the staurogram the earliest Christian images of Jesus on the cross.' [SEE HERE]

Above - On the “heads” side, Greek letters circle a royal crown and a chi (X) in the centre, saying “of King Herod.” Resembling a Christian cross, the chi stands for “christos.” On the “tails” side, palm branches again flank an incense altar. To Jews, King Herod made symbolic claims that he was their foretold messiah. Is there any historical evidence for interpreting these coins as claims by King Herod that he was the messiah? Yes! A text from the third century C.E. describes a Jewish cult that honoured King Herod as their messiah. The cult was founded by Boethus, High Priest of the Second Temple under King Herod. By using a Jewish coin to exemplify the non-messianic use of a staurogram, Professor Hurtado opens a window to pre-Christian Jewish messianism. However, the theory that Christians invented the messianic symbolism of staurograms in approximately 150 C.E. simply doesn’t hold water.

Coin Images were provided by the Classical Numismatic Group, Inc".
http://www.simchajtv.com/king-herod-a-messiah/

Below is the 'Herod coin' with the tau cross ...



This could all be interesting in relation to Constantine as a unifying emperor being aware of the staurogram significance as a Messianic device and as he claimed to be the claimant 'christ'  foretold in the poem by Virgil to legitimise his rule it could all signify something much deeper.

One wonders if the battle between King Herod and his family later with the historical Jesus has its roots in this contesting of Messiahship.