It's a trick question, isn't it? One could say that it never did, even though it was the standard-bearer for bringing Christianity to the world. Did Germany become pagan with the rise of the Nazis, or was it always partly pagan and incompletely Christian? The Goths were Christian,and the Vandals were Arian Christians, and displayed their faith by torturing the other types of Christians' priests. Clovis became Christian, and then his men the day after. So what did they mean by that come that Sunday? What would they have said, and at what point would a modern Presbyterian say OK, that qualifies. You're in the club.
I think we can find some hard measurements in the High Middle Ages of widespread Christian practice, a good bit of piety and sacrifice, and fairly good knowledge of basic doctrine. They tolerated a lot of physical cruelty, but all ages and places of mankind have done that. If you wanted to point to one very great difference of modern ethical sensibilities, noting that we are far less physically cruel would be a good first choice, and that is very recent. It is also fair to say that the Christians of the time defined their own Christianity largely in terms of having no Muslims, no Jews, no pagans, and no heretics nearby. Our definitions might be so different that neither would recognise the other. We would each find the other to be very great sinners.
The Puritans and Quakers who first came to America may have a good claim to serious Christian living, though that often did not include the one virtue we consider more important than all others today, tolerance. The German Pietists and Scandinavian Lutherans who came here...did all of those sincere believers drain the Christianity of European churches left behind? The state churches of Europe do not impress me with their sincerity and generosity now. If you dig into the history of any little village in Europe you quickly encounter pagan customs that were celebrated until quite recently and even unto today. These are sometimes merely high-spirited amusements on festival days, but folks will still say with a shrug that there seems to be something to it. I heard on a podcast last week of a legislator in Ireland requesting money for the repair of roads in his district because the government had made the mistake of building it right through a fairy ring, and they were digging it up and upending it every year.
I don't know that there was ever a golden age.