If you ask someone to do a job, you must first tell them what the job is and give a job description. Otherwise they might get partway into the job and decide this wasnt what they had in mind and quit. The last thing Jesus did before going back to heaven was to give every believer a job to do: make disciples. This book details what it means to be a disciple and gives a thorough job description so that we will know what to expect in fulfilling our duties. Matthew 5:12 says, And seeing the multitudes, he went up into a mountain: and when he was set, his disciples came unto him: And he opened his mouth, and taught them. The kingdom of God operates in a specific way and cannot be altered to fit our way of doing things. Jesus wanted anyone who accepted His call to know how to do it right. So He had to teach them.