This question is answered by the founder of the Watchtower Bible and Tract Society (Pastor Charles Taze Russell) here:
This is taken from chapter 7 of volume 1, the Divine Plan of the Ages, of his Studies in the Scriptures series.
You can find the thoughts of Bible Students on this question here:
Why must we take this from the Watch Tower, or the writings of Charles Taze Russell? Is not the Bible our source of Gods word?