One of the cornerstones of the theology of Jehovah’s Witnesses is the idea of two different natures and two worlds. It goes back to Russell himself, who unfolds it in the first volume of “Studies in scripture.” The two basic natures that exist are the earthly and the heavenly. To understand this thinking, it is important to understand that these two natures are separate. This means that an earthly human being cannot be a spiritual being  Studies in Scripture, vol. 1, p.206 . The two natures cannot be mixed  Studies in Scripture, 1, p.208 .
This division is of crucial importance for the formation of both Christology (the doctrine of Christ) and eschatology (the doctrine of the last things; here more specifically: The new world), and this is what this post is about.
Two worlds and the view of Jesus
When the two natures are totally separated, it means that Jesus cannot be both spiritual and human at the same time. This means that Russell developed a Christology in three phases: Before he was human, while has was human, and after his resurrection.
Before he became human, Jesus was a “spiritual nature.” The same is true of him after his resurrection. As a spirit being, one is immortal. Russell defines Immortal as “a state in which one cannot die,” as opposed to mortal, in which death is a possibility. When Jesus became human, he became fully human, and at that time had no spiritual nature. The transition from one state to another is what Russell calls nature transformation. The first transformation happened when Jesus became a man, and the second happened when he was resurrected.
Two worlds and the view of eternity
That dichotomy of reality, one a spiritual and one a physical reality, becomes fundamental to Russell’s understanding of the future. In the same way that there is now a physical and a spiritual world that is separate, so will it be in eternity. There is a small group, that has to live in the spiritual world with God and Jesus, and there is the large group that has to live on the recreated earth. The earthly group will continue to be earthly. The earthly group will be limited in the understanding of the heavenly things, and will therefore not strive for heavenly understanding.  See this post for details
Two worlds – a key to understanding the Bible
Since Russell, this division of reality has been fundamental to Jehovah’s Witnesses. As mentioned, it is reflected in the division of eternity, but it is also reflected in the fact that the New Testament is primarily addressed to those who are to be in heaven. Most of the theological concepts that are about salvation – eg grace, salvation, child of God, receiving the Spirit, church – are concepts that describe the group that is to be in heaven. They are also places where one part of the text are read about the group that is to be in heaven and another part of the text is read about the group that is going to be on earth.
A concrete example is the understanding of John’s Revelation chapter 7. [note] The following is taken from “Revelation, the great climax is near! p.113ff” [/ note] The chapter falls into two parts. In the first part, John hears about 144,000 of the tribes of Israel being sealed. In the second part (7: 9-17) we see a large white crowd. In the theology of Jehovah’s Witnesses, the 144,000 are a group that will live in heaven, and the great white crowd is the group that will live on earth. Russell himself uses the term “church” about the group that is to be in heaven.
A practical theological consequence of this division is that only the 144,000 participates in the sacrament of communion. Originally, it was also only they who were part of the 144,000 who were church leaders, but this has been abandoned as the group shrinks.