Lending money?
Today Islam still holds this as a majority view in many places (not universally). I'm simplifying it a lot, so I really recommend the book. There is a strong correlation between debt and guilt. In Christian doctrine, they're very interchangeable (forgive us our debts/sins as we forgive those who are indebted to/have sinned against us).
Guilt, Sin and debt are the same word in many ancient languages, including old English. Guilt and debt are still the same word in German.