The reason being is Christian emperors put a preference for bueareactaric position on Christians, in the same way Julian had a preference for choosing non-Christians over Christians. The effect of that is, if you're an ambitious bureacrat who wants to get good posts in the empire, you are going to convert to the religion of the guy handing out the posts, and do whatever else is necessary to put yourself in a good position to advance through the bureacracy.
Exactly! This way more and more male noble romans became christians. A religion of slaves and females (both without any political influence) so far.
Constantine was no christian. Forget his last minute baptism! Constantine always instrumentalized christianity. Nothing more and nothing less. But unfortuenately his wife and therefore his sons were christians. And from now on every sycophant in this empire became a good christian by very good reasons. Well he became, whatever this sycophant (also an educated roman magistrate) thought, a christian should look like, in front of the emperor.
Constantine and his successors also showed great interest in theological issues. They themselves invoked and presided councils discussing important theological questions. Of course, the intention of a roman emperor was, to control the situation, implement himself as leader of christianity, instrumentalize religion and finally agree upon the one and only roman opinion supported by the emperor. Centralism as usual.
Fortunately, Julianus stopped this farce. I am afraid, that the roman emperors would not be succesful in controlling christianity longterm.
Imagine, how much christianty could have been perverted, by these roman emperors and all these sycophants following the emperors. A romanized christianity, with a central authority and one (christian emperor) cult harmonized all over the world. Enforced by the gladius, if needed. Again as usual. Even if fully unchristian.
On the other side, I doubt, the roman emperors really could have been succesful to implement themselves as the one and only leader of chrstianity. I expect, that these damn bishops gonna start to undermine the absolute authority of the emperors.
PS: OOC: I even doubt that Theodosius The Little was a true christian. He was perhaps just another roman emperor failing dramatically to control the spirits he cited.