Sure, Christianity continued to influence some aspects of culture and society in some Western countries until about the Second World War -- maybe even up to about the 1960s in some places -- but it would be a stretch to refer to the influence of this sum of "somes" as Christendom or as characteristics of a so-called Christian country.
Christendom is behind us -- far behind us. Same goes for Christian nations. Furthermore, I doubt that I will ever live in a Christian nation or in any kind of Christendom in my lifetime. My son's lifetime? Who knows? Maybe -- but probably not. Same goes for his kids.
Bleak outlook? Well, it depends on one's perspective. If Christianity depends on things like Christian nations and Christendom, then the outlook is bleak indeed.
However, if one can separate Christ from nations and a grouping of nations, then the outlook brightens considerably.
There is no Christendom. There are no Christian nations. There is only Christ.
And if you stop and think about it -- really think about it -- you might begin to see this as a blessing, not a curse.