Change in fortune? In what way was the Christianity of the first three century on the way out?
How can its transmogrification into a state religion where one is a Christian for reasons to do with state citizenship be referred to as survival?
I pose these as questions in the general category of asking you to justify your theory that Constantine was instrumental in the survival of Christianity.
I never said that Christianity was 'on the way out' prior to the 4th century, so another of your endless straw men.
I'd have thought any idea, from among competing ideas, that became the subject of considerable government support would see it as good fortune: so in the 4th century Christianity benefited in a way that the the pagan alternatives didn't, which is perhaps why there aren't too many Temples of Apollo/Jupiter/Neptune etc in regular use down your way (or mine for that matter).
The later Roman Emperors, with the notable exception of Julian the Apostate, certainly gave Christianity a leg up and Constantine was a key player: after all, iirc, it was he who called the First Council of Nicea which was (and still is) fairly influential on Christian theology ('Nicene Creed', setting the date of Easter etc).