The riddle:

‘Can you name any moral action or ethical statement that could be made or performed by a believer but could not be made or performed by an unbeliever?’

The answer:

‘Here is my attempt to win my own prize. When Lech Walesa was starting his work in the Polish shipyards and the Polish Militia and the outer ring of the Polish Army were closing in on Gdansk, he was interviewed with his then fairly small group, and he was asked: “Aren’t you frightened, aren’t you afraid? You’ve taken on a whole powerful state and army – aren’t you scared?” And he said: “I’m not frightened of anything  but God or anyone but God.”
‘This came back to me, I thought, well, this meets my two criteria. It’s certainly a noble thing to have said, a distinguished thing to have said, and I certainly couldn’t have said it. So it does meet both my criteria.’

From here