He confessed, and did not deny, but confessed, "I am not the Christ."
And they asked him, "What then? Are you Elijah?" He said, "I am not." "Are you Elijah?" "Are you the Prophet?" And he answered, "No."
Then they said to him, "Who are you, that we may give an answer to those who sent us? What do you say about yourself?"
He said, "I am the voice of one crying in the wilderness: "Make straight the way of the Lord," as the prophet Isaiah said."
John 1:19-23 (NKJV)
To understand something, it is necessary to know what it is, and it can be helpful to also know what it is not. Understanding that water is something you drink and gasoline is something you put in your vehicle is necessary. Understanding that a mountain lion and a cat are different, is important to know when you are considering getting a pet. And understanding that someone who plays professional basketball is not going to do well trying to be a jockey riding race horses, helps someone know what work that person is better suited for.
When John the Baptist was questioned by the Jewish religious leaders about who he was, John told them who he was, but John also told them who he was not. John had an understanding about who he was, and what God had called him to do, and John also knew who he was not. By understanding both who he was and who he was not, John was able to do what God had called him to do. If John had not understood not only who he was, but who he was not, John may have had problems fulfilling God's plan for his life.
There are many Christians who struggle in life because they do not recognize and understand who they are in Christ, and they fail to understand what God has called them to do, and they do not know what He has not called them to do. As we develop and grow in our relationship with God, we can learn who we are, and we can also see who we are not. Knowing what God has called us to do, and also understanding what God has not called us to do, will help us be effective members of the Body of Christ, and fulfill God's will for our life.