mountainman

Is Michael Grant here using irony at the
same nexus in ancient history described
twice by Momigliano as a "miracle"?

"Trances and visions and hallucinations
were a feature of the age. Perhaps
Constantine had seen a rare cross-like
natural phenomenom, produced by the
sun. At any rate, whatever the explanation
Constantine was able to convince himself
that he had been granted a supernatural
experience."

p.354. The Ancient Historians (or via: amazon.co.uk) - Michael Grant

Constantine was able to convince himself
that he had been granted a supernatural
experience.

Is this not rather a strange way for an ancient
historian to describe the turning point in the rise
of christianity?

On 28 October 312 the Christians suddenly and
unexpectedly found themselves victorious. The
victory was a miracle - though opinions differed
as to the nature of the sign vouchsafed to
Constantine.

Toto

Michael Grant is a secularist, sometimes accused here by Christians of atheism. He is using the conventional formula nonbelievers use when they talk about claimed miracles in ancient history - that people believed that they had a supernatural experience. We all know that there are people today who believe that have they had a supernatural experience, even if they can't claim Randi's prize and secularists think that they are deluded.

But the word miracle has been so degraded by common usage and advertising that to say something was a miracle does not necessarily mean that the author thinks that anything supernatural was involved. (A common food on American shelves is "Miracle Whip" - an overly sweet substitute for mayonnaise.) I would think that adding "opinions differed" makes it clear that the author is not endorsing the supernatural nature of the miracle.

Other than that, you are not getting anywhere. There is nothing in this that states or implies that Christianity suddenly popped into existence when Constantine saw the "miraculous" sign.