If my understanding is correct, his veneration in the eastern Catholic Churches puts him in a similar category to Blesseds. Blesseds are people whose veneration the Church has permitted by some official act but not yet commanded for the whole Church. In the Catholic Encyclopedia’s summary, “if [the Church] only permits such [veneration], or if it binds under precept, but not with regard to the whole Church, [that] is a decree of beatification.”
source In this case, the official act permitting the veneration of Constantine, if my understanding is correct, is the act whereby various eastern churches were restored to full communion with Rome. It is my understanding that, as part of this reconciliation, they were told that they did not need to change their liturgical calendars. To me, that would seem to be an official permission to keep venerating the people on their calendars, including Constantine.
My above analysis is suspicious to me because there are lots of things I’m unsure of: has the Church officially said something permitting the use of eastern calendars? If it has, is that in Fact an equipolent beatification for the people on that calendar? It is only my own analysis that tells me it meets the definition given in the Catholic Encyclopedia. But I could definitely be wrong, and even if I’m right, the Catholic Encyclopedia could be wrong. Is it really true that a beatification is when the Church makes some act specifying that a particular person or group may be venerated? And what about the “or” in the Catholic Encyclopedia? It says that beatifications are when the Church permits someone’s veneration “or if it binds under precept, but not with regard to the whole Church.” I don’t think the latter example happened in Constantine’s case, but I Do think his veneration has been officially permitted because I think the Church has officially approved eastern calendars that include him. If that is correct and if the Catholic Encyclopedia is correct, then that appears to count as a beatification under their first definition: official permission to venerate an individual or a group.
If the above analysis is correct, then Constantine would be equivalent to a Blessed: his veneration is officially permitted, especially in eastern regions, and he may therefore be prayed to. Anyone who wishes to venerate a Blessed is permitted to do so, you don’t have to be in the particular region where they receive special veneration.
There are also several important differences between Beatification and Canonization, one of which I think is worth pointing out here: it is my understanding that beatifications are not infallible because they are not a command to the whole Church to venerate someone. When the Church commands the whole Church to do something we can be sure it is not commanding us to sin, and that is why I think some theologians consider canonizations infallible: the Church’s holiness is incompatible with the idea that it could require everyone under pain of sin to venerate someone evil. Beatifications, however, are Not a command made to the whole Church under pain of sin, they are rather an endorsement of veneration by a particular region, or a binding of only a particular region – and I suppose the Church can be wrong about such things. Thus, Bl. Constantine the Great may have his veneration Permitted, but until he is Canonized by the universal Church (or equipolently canonized) I don’t think we can be capital-c Certain that he is in heaven.
Based on the above analysis, I personally conclude that Constantine’s situation fits the definition of being a blessed. Therefore:
Bl. Constantine the Great, pray for us!
I hope that helps. God bless!