So John and Sherlock are shuffled into a church during a case. There's people already there and John and Sherlock are mistaken for patrons of the church. They have to join in confessions for some reason (it's rather hard to say no to such nice people).

Sherlock confesses his "sin" of "seducing" [insert person of high status in a community such as a priest, teacher etc] when he was a little boy.

and... he truly believes it was his fault.