The Central Idea of the Text is that Jesus warns His disciples they will be hated and persecuted because He has chosen them out of world (John 15:18-20).  There is no rational excuse for those who hate Jesus and reject Him as Savior.  Jesus’ words and works testified to His being the Messiah sent from God the Father (John 15:21-25).  The Holy Spirit, whom Jesus and the Father would send, would testify of Him (John 15:26), and believers who walk with Christ will bear witness as well (John 15:27).