In the book, he talks about three distinct groups of people who are leaving classic expressions of faith, and landing in significantly different places, Nomads, Prodigals, and Exiles.
Nomads--Make claims to a basic truths of the faith but don't engage in outward expression of the faith.
Prodigals--Have completely separated themselves from the faith in which they were raised.
Exiles--Hold closely the basic truths of the faith, engage in outward expression of faith, but in very non-traditional ways.
And what has lead to this array of departures? According to Kinnaman's book the biggest problem is that these youth who are raised in the church are not being equipped with the tools they need to make sense of the world in which they live through their faith communities.
For instance, although there are many scientists (myself included) and workers in the realm of science and technogy who have little difficulty seeing their engagement with science, medicine, health, and technology as an arena in which to express their faith, many young people are raised in the church with either an anti-science background (at worst) or little thought being given to how faith and science work together (more generally).
Aside from being antiscience, Kinnaman notes in his research that these young nomads, prodigals, and exiles also consider the church to be overprotective, repressive, shallow, exclusive, and doubtless. While Kinnaman shares these concepts with the data to back them up, he also paves a way for the church to navigate going forward to course correct amid these problems. For me, this was the most encouraging part of his book.
Why was it encouraging? Well, the reason was two-fold. The first reason is because it would be very easy for a researcher to simply just lay out the findings and walk away to let the reader engage with the data and figure it out. David doesn't do that. He cares so much about this issue that he is willing to share examples of bright spots amid these negative trends. The other reason I was encouraged by this book was more personal. I read the book and felt affirmed in the way in which I went about my work in vocational ministry on the campus of West Virginia University and in the city of Morgantown, WV while I worked for Great Commission Ministries at Chestnut Ridge Church.
I highly recommend his book; particularly for church leaders and others who are wondering why they are seeing a decrease in attendance among those in the 18-35 year old range in their congregations. I would also recommend reading it if you are a person in this age group period. It is well written, not preachy, and Kinnaman writes from the position of a fellow journeyer.