"There will always be skeptics who doubt Johnny's involvement in his wife's murder even though he was proven innocent."
"There will always be skeptics regarding the theory of Evolution."
"There will always be skeptics on the matter of the existence of God."
"There will always be skeptics who question what the CEO really did with the money that was donated."
The word skeptic means a person who "inclined to doubt something" (someone who doesn't believe something), and that something is normally an accepted fact,things that most people believe,evidence. I don't really know what your teacher is looking for so I suggest you make up your own sentence. But hopefully it helps. :)
