This is a guest post by Aesop
Once upon a time there was a boy who looked after the sheep for his village. Every now and again a wolf would come and eat one of the sheep, and some of the villagers would complain; but the boy enjoyed looking after the sheep, so he would say “What do you mean, a wolf? I didn’t see a wolf. Anyway, I don’t think wolves even exist round here; some people just make up stories about wolves because they don’t want me to look after the sheep.” After a while all the villagers believed him, and they were relieved that they didn’t have to worry about wolves any more.
Then one day a really big wolf with sharp teeth and a glint in his eye came to the field, and said “little boy, I’m going to eat YOU!” So the boy shouted “Help! Help! There’s a wolf and he’s going to eat me!” And all the villagers said, “Oh sure, you expect us to believe that? Just concentrate on your sheep and don’t worry us about wolves.” So nobody would help the boy, and he was eaten by the wolf.
Liars are not believed, even when they speak the truth.
Or:
Don’t spend twenty years telling everyone that antisemitism is a Zionist lie designed to stop people criticising Israel, and then act surprised when your movement turns out to be riddled with antisemites.