"There's a famous scientist who agrees with me!"

 

"My priest says it's true!"

 

If you are going to refer to the opinion of someone as evidence for your argument, it is important that their opinion is relevant. And this is only the case if they are qualified in the particular subject you are discussing. For example, if you are discussing quantum mechanics, it is no use appealing to the authority of an expert biologist. No matter how intelligent they are, or qualified in their field, they are not automatically qualified to talk about any other given subject. 

 

Also, even if the person is qualified to give such an opinion, that doesn't necessarily mean they are correct. 

 

There are some subjects for which no one is qualified to speak, such as the existence and behaviour of the supernatural. It is an area which by definition cannot be studied, so to be an "expert" in it is meaningless.