How do we know that false teachers exist according to the Bible?

Jesus addresses the presence of false teachers in John 10, referring to them as thieves and robbers who do not enter through the legitimate gate, which is Himself. He warns that true sheep will not follow these impostors because they do not recognize their voice. Additionally, passages like Jeremiah 23:21 and 14:14 echo this reality, where God declares that many prophets are self-appointed and not sent by Him. This serves as a crucial reminder for believers to discern truth from error in their spiritual leaders.