This book was written to teach Christians to understand the foundations of the book of Revelation to see how it is put together by using the Old Testament as a shadow of the new. In the Old Testament, we are taught about the harvest of grains, barley, wheat, and grapes; and in the new, we are taught about the harvest of souls. The souls are the Christians, Israel, and Gentiles. Each harvest has three parts—firstfruits, main harvest, and gleanings. This is the same for grains and souls.
Christ is the second Adam. This means we can look at Adam as a shadow of Christ. Christians are part of the body of Christ, as God took a rib from Adam next to his heart. God will take the firstfruits from the body of Christ, the church, in the first rapture. Then he will take the main harvest, the body of Christ, in the second rapture.
Bible Reference Joel 2:16