Like most things it can be different things at different times :-
* it-1 p. 923 Genesis, Book of ***.
It sets the theme for the Bible, namely, the vindication of Jehovah’s sovereignty and the ultimate fulfillment of his purpose for the earth, by means of his Kingdom under the promised Seed.
*** jv chap. 18 p. 283 "Seeking First the Kingdom"
***THE principal theme of the Bible is the sanctification of Jehovah’s name by means of the Kingdom
*** ip-1 chap. 17 p. 215 par. 1 "Babylon Has Fallen!"
***the Bible has a major theme—the vindication of Jehovah’s sovereignty by means of the Messianic Kingdom government
***
w61 6/1 p. 324 Do You Teach Your Children the Counsel of God? ***The Bible has a theme, the kingdom of God by means of which God will prove to all mankind not only that he exists but also that he is the Most High and the One perfect in wisdom, justice, love and power.
*** w66 3/15 p. 179 par. 16 "Increasing in the Accurate Knowledge of God"
***Foremost of these is the theme of the vindication of Jehovah’s name by means of the Kingdom Seed.
*** w71 4/15 p. 230 The Bible—the Creator’s Guide for Man ***
throughout its pages the Bible focuses on the theme of the inspired writings—the kingdom of God in the hands of Jesus Christ as the means that Jehovah has provided for carrying out his purpose