God tells us His ultimate purpose and will in Ephesians 1:9-10: Having made known unto us the mystery of his will, according to his good pleasure which he hath purposed in himself: That in the dispensation of the fulness of times he might gather together in one all things in Christ, both which are in heaven, and which are on earth; even in him:
He will eventually gather all things together in Christ in two distinct places.
Starting in Genesis 1:1 we have the heaven and the earth. In the end of the scriptures in Revelation 20, we have a new heaven and a new earth.
God has one purpose: All things are gathered together in Christ. But He will do this in two places: heaven and earth.
Rightly dividing the word of truth is simply understanding which books are written for the earthly part of the kingdom, and which are written for the heavenly part of the kingdom.