Soul Harvest (Left Behind #4) by Tim LaHaye and Jerry B. Jenkins

Rayford Steele and Buck Williams fear they are alone. Each has survived the wrath of the lamb, a global earthquake in the twenty-first month of the tribulation. Neither knows the other is alive, and each is frantically searching for his wife as the devastated world hurtles toward the Trumpet Judgements and the great soul harvest prophesied in Scripture. With the fulfillment of more Bible prophecies, hardly any skpetics remain. Even the enemies of God now know whom they are fighting, and the world is forced to take sides...