How can we be sure Rutherford used the 6,000 year theory? Because of the dozens of times he mentioned it in Watch Tower literature. For instance, Rutherford wrote in the January 1, 1921 Watch Tower:
The poor groaning creation has suffered oppression for more than six thousand years. They are groaning and travailing in pain, waiting for the manifestation of Messiah’s kingdom.… The Scriptures clearly show that this old order began to pass away in 1914 with the coming of the World War, and that it is rapidly disintegrating. …that the old world is passing away; that Messiah’s kingdom is at the door; and that millions now living will never die.
In the March 14, 1923 GOLDEN AGE, this point was made:
…it can be confidently stated now that millions living at this time on the earth will never die. We are in the transition period. Instead of becoming discouraged, the student of prophecy should look by faith beyond this dark night to the new day, the Golden Age that is just dawning. The whole plan of God relative to man, which covers a period of 7,000 years, reaches a climax in the restoration of man and his perfect home on earth.
In Rutherford’s 1924 book, THE WAY TO PARADISE, he made this point:
Christ’s kingdom is to undo in one thousand years all the evil brought about in the previous six thousand years.