11 November: Diocletian (briefly emerging from retirement), Maximian and Galerius meet at Carnuntum (near Vienna) to resolve the embroiled tetrarchy: Diocletian returns to retirement rather than reclaim power as senior Augustus; Maximian is to also return to retirement; Maxentius is declared a usurper; Galerius appoints a new Augustus, Licinius, and offers Constantine and Maximinus Daia the title filius Augustii.
Basilica of Maxentius/Constantine
Re: the Agonalia
Tom raises one of the (religious) ambiguities that may always surrounded Constantine. I have read some reference to the popular worship of Mithra (spelling? - an eastern sun cult I believe) during Constantine's time. For example, while Constantine was one of the junior emporers ruling from Trier over Gaul, Britian and Spain, the overall political crises engendered by the usurpative Maxentius in Rome brought together the retired emperors of Diocletian, Maximian (Maxentius' father) and Galarius (eastern Augustus) for a meeting near Vienna. There is some existent inscription relating to that meeting suggesting that offerings to Mithra were made. I don't think that Constantine was at that meeting, however.
My feeling is that both Constantine and Helena were very interested in Chistianity, and perhaps believers, before Constantine's vision in 312. If they were, however, they had to keep it very much to themselves. The retired emperor Maximian, who resided within Constantine's court in Trier (c. 308-311) was an ardent persecutor of the Christians while he co-reigned with Diocletian. One has to look carefully at Constantine's early political position as emperor to see that it was precarious, and professing even a tolerance of Christianity before his position became stable might well have been political, if not literal suicide.
Personally, the more I research this particular history, the more facinating it becomes. For example, check out the uncanny family relations between Constantine and Maximian, and you will realize that none of the Constantine-Helena story is easy.
Constantine "practiced" Christianity as of 312, but was not baptized until a few days before his death in 337. Constantine's remaining an unbaptized Christian for most of his life is precisely the issue that raises all the ambiguity surrounding his "faith".