In the gospels of Matthew, Mark and Luke, they have the "Kingdom of God" as the heart of Jesus's message. However, in John he almost never uses the term "Kingdom of God". John translates "Kingdom of God" into a phrase that is hard to translate into English. It is usually written as "eternal life". Thus eternal life is interpreted to mean life in heaven after you die. Let's see if I can show you a different interpretation.
If eternal life doesn't mean life after death, what does it mean? In the last half of John's Good News, Jesus simply reduced the phrase to "life" or "life to the fullest". Near the end of John's story, chapter 17, verse 3, Jesus states a fascinating truth and it, I believe, gets us close to a definition: "This is eternal life: that they may know You, the only true God, and Jesus Christ whom [God has] sent." Here eternal life is a verb knowing. Knowing is an interactive relationship or simply put getting to know one's spouse, a friend, a fellow student, a co-worker, etc. So this means we develop a two way relationship with the only true God and His messenger, Jesus Christ. Doesn't this relate to a kingdom: an interaction between the King and His subjects and the subjects with each other and so on?
The Greek phrase John uses for eternal life means "life of the ages." This tells us that Jesus was telling us to live a life that is radically different from the way people are living these days. It is to live a life that is full and overflowing and is centered on and an interactive relationship with God and with Jesus. To put it simply, "Live an extra ordinary life to the fullest, centered in a relationship with the Living God.