There was a 'big change' once Christianity was 'accepted' by the Roman Emperor and the state became a so-called 'sponsor' of the religion under Constantine.
Imagine that the basis for his 'conversion' is that he visioned a "Cross" to lead him to victory in the process of killing (battle). The one who died for all of humanity in meek and peace, becomes the inspiration to kill during battle!
As he was for King David also.
That idea gets tossed around a lot, but nobody can come up with any specifics about exactly when the "big change" happened or who the players were, etc.
The main controversy at the time was Arianism. Constantine himself was sympathetic to the Arians, but tolerant toward orthodoxy. No mainstream Christians today accept Arianism, so (unless you're a JW) the rejection of Arianism couldn't have been the "big change" you're talking about. So what was it, and when did it happen?