Not all those who wander are lost, J.R.R. Tolkien observed. The corollary is that not all who lead know where they’re going. Both these statements are true of Joshua Harris, the former pastor and author of “I Kissed Dating Goodbye” (1997), who acknowledged on Instagram last week that “by all the measurements that I have for defining a Christian, I am not a Christian.”

Mr. Harris’s announcement carried strange poignancy for millennials who grew up in evangelical churches. In the 1990s, when they were teens, the mood was countercultural....