When this Gospel was first written, the intention was to project the power of the Emperor through Christ to the Flavians, so Christ was seen as the one with ultimate authority.

Several centuries later when Constantine came around, Imperial authority had diluted to the twelve Bishops established by Diocletian, who each had the power of an Emperor within their own realms. For Constantine to both reflect his reality of power and also co-opt the Bishops into his scheme, he would have needed to insert just such a scene, where the twelve Bishops become the twelve Apostles (or Deputies), also able to cast out demons (meaning invaders, rebellions, etc) by their own right.

In Early Church myth, the Apostles were the first Bishops of the various dioceses, so this is a pretty clear connection.

It’s also of note that the word for ‘preaching’ here, κηρυσσειν, is used a couple of times prior to this point in Mark: at 1:14 and again at 1:38-39, it described Rebel Jesus’s behavior, and at 1:45 that of the leper. But not Anointed Jesus. Will have to keep a note on the further use of this word, but here it appears to indicate the behavior of a messenger, and not that of a source of authority.

This verse starts with “making the Twelve”, εποιησεν δωδεκα, and the phrase is repeated at the end of the next verse. This seems to bracket the specific powers of these Apostles: they were to “preach”, or speak for the highest authority; they were to “cure infirmaries”, or improve morale; and they were to “cast out demons”, or soothe any rebellious impulses.