There is another story which addresses the donatism question...
An old granny used to see how the priest would arrive at church, bowed down with the weight of a body which he had to carry on his back everywhere he went. He had murdered someone. When he came to the church doors the body always slipped from his shoulders and he entered the house of God and served the holy Liturgy. When he left the church he again picked up the burden of the dead body at the door.
That has to be the best illustration I've ever heard for this. Thanks for sharing.