After the Black Death.

As you exit your humble hut, you see some groups seeming to.... Whip their backs with whippers filled with nails, they are doing it for repentance from God.

"God! Please forgive us, we are harming ourselves for thine merciful and forgiveness!" The man shouts whilst he continues severely to whip his own back with the whip filled with nails. It almost looks like the one called "Cat O Nine Tails".

And some other groups follow.... To literally harm themselves for the sake of God's forgiveness....