It was intact in 1500, Europe all Catholic.
The awful Wars of Religion that followed the Reformation did not destroy it wholly, since they were carried out precisely by competing visions of Christendom. Even the execution of Charles I in the mid 1600's was significantly about religion.
One shocking precursor of its demise is the Franco-Ottoman alliance in the 1530's, where a Catholic kingdom allied with the Muslim Turks to fight against the Holy Roman Empire. Talk about "perfidious Albion"...the French who created that phrase certainly got a head start on it.
Church, State and culture --both popular and elite--remained heavily interlaced, even indistinguishable, until the 1700's, although Descartes and Newton published in the 1600's.
A watershed is 1789, I think, the French Revolution and its Napoleonic aftermath. There were revivals after that, but the trajectory ever since was downward. They were loading the gun in the century prior, shot the fatal bullet in 1789 and then the patient finally died several decades later.
I think it was pretty much dead by the mid-19th century, although people could still speak sentimentally of Christendom when they meant Whites (or in Churchill's case, non-Nazi Whites).
The majority of western Europeans --in the home continent and elsewhere-- remained Christian in some sense long after that, but both the intelligentia and the States were operating on a quite different set of assumptions. While Christendom existed, the Church --in its various forms-- was a power that was in some sense required to legitimate the State, even if its actual role was as chaplain, as in Anglican England or Orthodox Russia. But its power has been merely sentimental since the 19th century.
The Liberalism which is now completely hegemonic in the West is the toxic grandchild of Christendom, more proximately the evil love/hate-spawn of the Enlightenment and the Romanticism which reacted against it.
Christendom was destroyed partly by its own internal conflicts but partly by the inherent
Interesting to read the relationship of faith and reason, visionaries and skeptics, throughout Western history using Samuel Huntington's typology:
"must learn to distinguish among our true friends who will be with us and we with them through thick and thin; opportunistic allies with whom we have some but not all interests in common; strategic partner-competitors with whom we have a mixed relationship; antagonists who are rivals but with whom negotiation is possible; and unrelenting enemies who will try to destroy us unless we destroy them first."----