A.B. Simpson emphasizes the principles of 'Aggressive Christianity,' which is characterized by an absolute faith in a supernatural God and an intense spiritual earnestness. He argues that true faith leads to unselfish and aggressive work for God, urging believers to reach out to neglected fields and share the Gospel with those who have not heard it. Simpson highlights the importance of a deeper faith as the source of missionary work and the necessity of a spirit of love that compels Christians to care for the lost. He calls for a collective effort to evangelize the world, stressing that aggressive Christianity is essential for fulfilling the Great Commission and addressing the dire spiritual needs of humanity.