Even though the twelve disciples had the privileged access to Jesus that they had, they were not immune to the influence of Jesus' enemies. Though they want to follow him, still they have difficulty "getting" what Jesus and the Kingdom of God is all about.
Jesus' warning to them, "Beware the leaven of the Pharisees," is a warning to us, too. As long as we live in the present world, we are not immune to its antichristian influence. We must be careful to keep our minds and hearts full of the true word of the Lord, Jesus.