There is a line of guilt/sin that connects Hester and the Reverend Dimmesdale. Hester, guilty of adultery, holds the secret of her lover's sin in her heart while Rev. Dimmesdale holds his secret sin within himself and is tortured by guilt.
Secret sin is the key to the actions of each of the three main characters. This line of guilt/sin extends later to Roger Chillingworth who seeks the secrets in the tortured heart of the Reverend Dimmesdale.