Jesus knew that when Peter declared Jesus to be, “the Christ of God” that Peter would be changed by this declaration.
Jesus taught that we are changed as much by what we say as what we hear. Mark 7:15 (NIV) “Nothing outside a man can make him ‘unclean’ by going into him. Rather, it is what comes out of a man that makes him ‘unclean.’” “What comes out of a man”—what a man speaks is what makes him clean. We are changed by the truth when we speak the truth. When Peter declared Jesus to be the Christ, he believed ever more firmly that Jesus was the Christ.
This is why the Bible makes a big deal about “if you confess with your mouth.” (Romans 10.9) It is not that God needs to hear. It is not even that others need to hear. It is that you need to say. When you confess the truth with your mouth, you are changed by that truth.
Jesus’ brother, James, spoke about that. He said the tongue (what you say) is like the rudder of your life. Compared to the ship, the rudder is small, but it turns the whole ship. A bit in the mouth of a horse is small compared to the horse. But, it can turn the whole horse. A match is small compared to a forest. But, a well-placed match can set the whole forest on fire. Well placed words can set a whole church on fire. The words you use set the direction of your life.
I think if James were writing today he wouldn’t talk about bits and rudders, he would talk about steering wheels. Your mouth is the steering. You can steer your whole life with your words. Maybe he would talk about a computer. Your mouth is like the keyboard and mouse on your computer. You control the computer and tell it what to do through the mouse and keyboard. So, you control your life by controlling the words.
James had just warned that not many should be teachers. Perhaps he had teachers on his mind when he spoke of the idea that the tongue controls the life. Perhaps he was hinting to the teachers that if you can get people to say the truth, they will be changed by the truth they confess. We are changed more by what we say than what we hear.