Thomas Brooks emphasizes that sin's dominion over a person is characterized by its absolute authority and control over the soul, akin to a king ruling over his subjects. He explains that when a person willingly and habitually submits to sin's commands, it indicates that sin reigns in their life. Brooks warns that this dominion manifests through a person's delight in sin, their insistent pursuit of it despite consequences, and their active efforts to fulfill sinful desires. He highlights that true freedom from sin comes through grace, as stated in Romans 6:14, and encourages believers to resist the reign of sin in their lives.