This book anticipated, and may have helped create, the end-of-millennium wave of apocalyptic imagination that the Left Behind series commercialized so spectacularly. Best read as a stand-alone (I read, but did not enjoy and barely remember, the sequels coauthored with Robert Wise).
My head is fuzzy on the details, but as I recall the plot was based on a fairly literal take on Revelations/Apocalypse, and on biblical prophecy in general. I kept the book on my shelf for years because of the Appendix, which included a mind-bending conspiracy-theory-esque reading of the prophetic timeline that seemed, perplexingly, less and less brilliant the older I grew.
On the whole, this book is probably more valuable for its glimpse into the American Evangelical/Fundamentalist mind and the state of popular hermeneutics at the end of the twentieth century than for its story.