At the start most of the countries in Europe were Christian kingdoms. At the end they were either secular republics or moving that way. Same way with the great unification wars of the mid 1800’s.
It is real hard to pin point where the thread was lost. So hard that I sometimes wonder if the “ideal” ever really existed.
There were many blame points — the Venetians turning the fourth crusade against Constantinople, the English and French supporting the Turks against Russia in the Crimean war (but for that, Constantinople could have been restored) and also WWI. A lot of this was really England’s fault.
Of course the ideal existed. It was a thousand year Golden age of Christianity, 4c - 13c. We are still coasting on its cultural achievements, poor orphans.