In 2nd Peter, Peter appears to be defending the truth and denouncing false teaching. In the second half of the chapter, Peter is able to testify that he was an eyewitness to the truth of Jesus’ ministry. He knows the message of Jesus isn’t something people made up for their own benefit, but that it came from God through Jesus.
We have not heard Jesus’ teaching directly from Him, but we have received it from the Bible. The Bible is the source we rely on today. When confused about what is true or false about God, we can avoid falsehood by knowing the truth well.