Already happened.
In USA Christians, depending on your definition of the term, are still arguably a majority.
In Europe and the rest of the West Christians are a small and shrinking minority, with possible exception of a few countries like Poland.
The West is now a post-Christian civilization. We have not yet seen how well that will work out in the long run.
I'm not optimistic.
From its original sources, it’s clear that Christianity never made a claim of being able to win the majority of the world. If, in fact, it did, that would make certain biblical statements look foolish. We can’t say on any basis that we deserve to live the mortal segment of our lives in a Christianized world. But when God makes a promise, God keeps it, and we are not foolish to believe in that.