I believe all thoughtful evangelical Christians should read and ponder what he says. Only time will tell if he is right, but even if some of what he predicts does not happen he still has said some things that need to be said and responded to.
Inhabitatio Dei has a good synopsis of Spencer's The Coming Evangelical Collapse as follows:
1. Evangelicals have identified their movement with the culture war and with political conservatism. This will prove to be a very costly mistake. Evangelicals will increasingly be seen as a threat to cultural progress. Public leaders will consider us bad for America, bad for education, bad for children, and bad for society.
The evangelical investment in moral, social, and political issues has depleted our resources and exposed our weaknesses. Being against gay marriage and being rhetorically pro-life will not make up for the fact that massive majorities of Evangelicals can’t articulate the Gospel with any coherence. We fell for the trap of believing in a cause more than a faith.
2. We Evangelicals have failed to pass on to our young people an orthodox form of faith that can take root and survive the secular onslaught. Ironically, the billions of dollars we’ve spent on youth ministers, Christian music, publishing, and media has produced a culture of young Christians who know next to nothing about their own faith except how they feel about it. Our young people have deep beliefs about the culture war, but do not know why they should obey scripture, the essentials of theology, or the experience of spiritual discipline and community. Coming generations of Christians are going to be monumentally ignorant and unprepared for culture-wide pressures.
3. There are three kinds of evangelical churches today: consumer-driven megachurches, dying churches, and new churches whose future is fragile. Denominations will shrink, even vanish, while fewer and fewer evangelical churches will survive and thrive.
4. Despite some very successful developments in the past 25 years, Christian education has not produced a product that can withstand the rising tide of secularism. Evangelicalism has used its educational system primarily to staff its own needs and talk to itself.
5. The confrontation between cultural secularism and the faith at the core of evangelical efforts to “do good” is rapidly approaching. We will soon see that the good Evangelicals want to do will be viewed as bad by so many, and much of that work will not be done. Look for ministries to take on a less and less distinctively Christian face in order to survive.
6. Even in areas where Evangelicals imagine themselves strong (like the Bible Belt), we will find a great inability to pass on to our children a vital evangelical confidence in the Bible and the importance of the faith.
7. The money will dry up.
Spencer predicted that, as a result of these trends: 1) There will be a migration of many former evangelicals into Catholicism and Orthodoxy looking for historic continuity and spirituality; 2) Pentecostalism and the influence of forms of Christianity from the global south will make increasing impact in the West and form new and different sorts of churches; and 3) There will be a growing and significant house church movement.
Read and think - and Pray.