How do we know that false doctrines are dangerous?

False doctrines are described by Jesus as 'leaven' in Mark 8:14-21, illustrating how they can infiltrate and ultimately alter a person’s faith. These teachings often appear innocuous at first, gaining ground bit by bit, which makes them particularly insidious. Once accepted, false teachings can lead believers away from pure truth and into error, potentially compromising the core of their faith. Historically, the Church has suffered more from internal falsehoods than from external persecution, highlighting the urgent necessity for Christians to remain vigilant against any form of corrupt doctrine.