“discussed it among themselves.” The Jewish rulers were lying to Jesus, and he knew it (cp. Matt. 21:32). John 2:25 says that Jesus knew what was in people. The Jews believed that John’s baptism was from man and had no divine authority whatsoever, but they would not say so publicly. This kind of thing goes on all the time in religion, business, and politics. People lie to get an advantage. That is why we have to be “wise as serpents” and walk by revelation if we are going to do well in the world.
Jesus did not want to answer the question that the Jews asked about where he got his authority. He knew they would only use the information against him. He also knew that if he asked them the right question, in this case about John, he could stop their attack, which is exactly what happened. We have to follow Jesus’ example and realize that many people will use what we say against us, so we have to rely on God to know what is really going on in the hearts of people and what we should or should not say.