Arthur Mitchell (John Lithgow) is the serial killer known as The Trinity Killer on "Dexter." He appears to have the ability to hide his secret life and the monster within -- a quality that fascinates Dexter Morgan (Michael C. Hall), as it is something he struggles with himself. On the surface, Arthur appears to be a loving family man, though he has been killing for more than 30 years. Dexter discovers this and instead of killing Arthur right away, chooses to study him in an attempt to learn from him. The decision to put off killing Mitchell comes back to haunt Dexter when Trinity kills his wife, Rita (Julie Benz).
"Are your garments spotless? Are they white as snow? Are you washed in the blood of the Lamb?"
"Hello, Dexter Morgan."