Pastor Robbey's sermon "Their Going to Know" stresses the significance of love in a believer's life, using John 13:34-35 as a central verse. He emphasizes that genuine love is demonstrated through action, self-sacrifice, and humility, not just emotions. By highlighting love's transformative power and connection to salvation, the sermon challenges believers to exhibit love in their daily lives as a true reflection of their faith. Ultimately, the message underscores that love, when lived out authentically, has the capacity to impact both believers and the world around them.