John goes to his friend Peter and says...

John: "i'm sleeping with the priest's wife. Can you hold him in church for an hour after mass for me?"

Peter doesn't like it, but being a good friend he agrees.

After mass, Peter starts talking to the priest, asking all sorts of stupid questions, just to keep him occupied.

Finally the priest snaps and asks Peter what he's really up to.

Feeling guilty, Peter finally confesses to the priest.

Peter: "My friend is sleeping with your wife right now, so he asked me to keep you occupied."

The priest smiles, puts a brotherly hand on Peter's shoulder and says..

Priest: "You better hurry home now, my wife died a year ago"

edit: formating