Okay, here's a saying that works in principle, but not in real life: preaching to the choir. Now, the saying is supposed to reflect the wasted effort of trying to convince someone who already shares the belief, and you can see how it works. After all, the choir in a church is already supposed to be versed in the biblical message the preacher is sharing with the congregation.
But in real life? No. Most of the folks in the choir are just there for the gig, or because their town doesn't have any other choirs for them to be in. Those singers are probably the people who need the sermon the most!
Of course, since they're in the choir, they're not listening. They're too busy struggling with their photocopied music or chatting with their neighbor. That's just how it is when you're a singer. I know--I've contemplated attending church just to join the heavenly choir, and I'm a dyed-in-the-wool pagan!
Hmmn, dyed-in-the-wool ... I wonder what I could say about that phrase?