You need enough examples of the right kind to sway the audience. It is conceivable to find someone with some kind of credentials who will say just about anything. So if it is possible to find someone who will challenge a claim, the next step should be to examine the credentials and the biases of the authority. Y our own experience has probably taught you already that not all experts are trustworthy. You must read carefully and critically.
Experts do make mistakes.
For many people, religion and religious leaders are among the most trustworthy of sources on any issue. Yet history is littered with example of how a good thing can be used for the wrong reasons. Recent history gives us the example of Jim Baker. Not so recently we have the example of Copernicus. I n 1543, Copernicus proposed the idea that the Earth was only one of several planets which orbit the Sun in his landmark book De Revolutionibus Orbium Caelestium .