a classic....

 Murphy showed up at Mass one Sunday and the priest almost fell down when he saw him. He had never been to church in his life.

 

After Mass, the priest caught up with him and said, "Murphy, I am so glad ya decided to come to Mass. What made ya come?"

Murphy said, "I got to be honest with you Father. A while back,
I misplaced me hat and I really, really love that hat. I know that
McGlynn had a hat just like mine and I knew he came to church every Sunday. I also knew that he had to take off his hat during Mass and figured he would leave it in the back of church. So, I was going to leave after Communion and steal McGlynn's hat."

The priest said, "Well, Murphy, I notice that ya didn't steal
McGlynn's hat. What changed your mind?"

Murphy replied, "Well, after I heard your sermon on the 10
Commandments, I decided that I didn't need to steal McGlynn's hat after all."

With a tear in his eye the priest gave Murphy a big smile and said;
"After I talked about 'Thou Shalt Not Steal 'ya decided you would rather do without your hat than burn in Hell?"

Murphy slowly shook his head. "No, Father, after ya talked about
'Thou Shalt Not Commit Adultery', I remembered where I left me hat."