A.B. Simpson emphasizes the importance of embracing one's calling and recognizing that life's changes and trials may be part of God's divine plan for our lives. He illustrates this through the example of Aquila and Priscilla, whose relocation led them to meet Paul and ultimately serve Christ. Simpson encourages believers not to complain about their circumstances but to see them as opportunities for God's purpose to unfold. He reminds us to abide in our calling and make the most of our everyday duties, trusting that God is at work in our lives.