In what reason shows, they must be. In what revelation purports to reveal, they cannot be. So a person is left with what Jesus said of God. Only a divine one can speak authoritatively about God. But we mortals can be deceived by greater and more evil intelligences. So we get this from St Paul :
Galatians 1:8
But even if we or an angel from heaven should preach a gospel other than the one we preached to you, let him be eternally condemned!
DO I personally believe Joseph Smith received a revelation ? Yes, it is the easiest explanation (or as is said on here ad nauseam 'the most plausible') but the further question is, Is it from God ? No, I don't believe that for a minute.
"false teaching there will be"
IF Jesus wasn't the final revelation how do we know any revelation is ?