Okay, so there’s not one general consensus, but a few theories.

There are three semi-contemporary sources about Constantine and a vision, the first being before the Battle of the Milvian Bridge (the way it’s portrayed in most modern stories). In a panegyric of Constantine from 310 (anonymous), he saw a vision associated with the Temple of Apollo, which was very much in line with his iconography linking him to Apollo/Helios/Sol Invictus. Some historians think this idea of a vision is where the subsequent visions mentioned are drawn from.

The next source, written at roughly the same time as the Battle of the Milvian Bridge is Lactantius’ de moribus persecutorum, and in this version, Constantine has a dream rather than a vision. This is where we get the account of Constantine being told to draw a symbol on the shields of his men (interpreted by modern historians to either be the chi rho (first two letters of Christ in Greek), or four X overlapping, another early Christian symbol). Lactantius as an author is fairly brief in his descriptions of things, so the fact that he mentions this is notable. But he also takes every opportunity to hype Christianity and Christian things (he’s writing just at the cusp of Christian acceptance by the wider society, and certainly felt he had to legitimize it to the more aristocratic (and literate) parts of society, which were more often pagan at this point), so any modern reader should keep that in mind.

Then there’s Eusebius of Caesarea’s famous account of Constantine seeing a cross in the heavens with an inscription that says “By this sign, you will conquer.” Eusebius seems conscious of how this might sound, so he prefaced it by insisting that Constantine himself (whom he had only met once or twice before) had told this story himself and swore it was true. Eusebius is often seen by historians as more pro-Christian, pro-bishop (of course, the people who interpret Constantine’s vision, are bishops; Eusebius is a major bishop in Judaea) panegyricist than a historian. He embellishes and highlights things that are important to him and his agenda more so than writing historically relevant (or physically possible) events. In my opinion, the pendulum has swung too far away from Eusebius as a historical source, but this is still good to keep in mind when considering Eusebius as a source.

To say that something like this is historical is difficult to manage given the inherent religious connotations of a divinely inspired dream/vision. The most scientific explanation I’ve found is attempting to describe what Constantine saw as a solar halo, which is a real, albeit rare phenomenon.

It seems likely that regardless of whether this actually happened or not, Constantine and his biographers were very invested in mentioning this type of story, and it was a part of the way Constantine wanted to portray himself and his Emperorship in relation to his newfound religion (or religious tolerance).