Great scriptures but to me he didn't finish the paragraph, and in the following verses you are given the keys to this. here is the rest of the paragraph verse 7 being the key:
John 15:6 (KJV) If a man abide not in me, he is cast forth as a branch, and is withered; and men gather them, and cast
them into the fire, and they are burned.
John 15:7 (KJV) If ye abide in me, and my words abide in you, ye shall ask what ye will, and it shall be done unto you.
John 15:8 (KJV) Herein is my Father glorified, that ye bear much fruit; so shall ye be my disciples.