Simply put, the kingdom of God refers to the realm in which God reigns. In fact, it might be best in many instances to translate the phrase “the kingdom of God” as the “reign of God.” This helps us to recognize that the kingdom of God exists wherever God reigns.
With the coming of Christ, God’s reign has begun.
Now, it is essential to note that the coming of Christ marks the beginning of God’s reign upon the earth! This is why the Gospel of Mark announces, “Jesus came into Galilee, preaching the gospel of God, and saying, ‘The time is fulfilled, and the kingdom of God is at hand; repent and believe in the gospel’” (Mark 1:14-15).
One of the reasons why many Christians have a muddled view of the kingdom of God is because we have come to view all of reality in light of two separate spheres: an upper and a lower sphere.
Allow me, in this post, to delve a bit deeper into the modern thinking in order to show how it has negatively affected our understanding of the kingdom of God.
In this line of thought (enlightenment, or Epicurean dualism), the physical realm (the lower, earthly sphere) stands in sharp contrast with spiritual realm (the upper, spiritual sphere).
Though many Christians correctly conceive of the kingdom of God in contrast to the kingdoms of the world, they, unfortunately, assume that when the Bible refers to the kingdoms of the world it means the physical realm, and when it refers to the kingdom of God it means the spiritual realm.
The problem is that in the modern thought, the physical world is considered the realm of scientific knowledge. In this realm, knowledge is gained by means of observation and experimentation. This means that the truths gained here are considered objective truths and are believed to be universally valid and binding on everyone.
In the modern thought, the spiritual world, by contrast, is the realm of private matters: such as, religion and faith. By its very nature, the spiritual world is considered to be inaccesible to science and scientific knowledge; after all, it is beyond observation and experimentation. Consequently, the truths gained in this realm are considered to be purely subjective and relative to each particular individual and/or group.
Unfortunately, the incursion of enlightenment dualism on Christian thought is abundant.
It is commonplace for Christians to conceive of God as though He is “up there” and distant. We often pray to God, who is up there, and we hope that He hears us. Even more so, we hope that perhaps He might come down here and answer our prayers.
One of the problems with this radical separation of the physical and the spiritual is the conviction that the physical realm is sometimes considered evil. Many Christians hold to a belief that the matters of this world are not important.
The Scriptures, however, reject such a radical distinction between the physical and the spiritual. After all, according to the NT, the New Creation entails the resurrection and restoration of the old creation. Paul notes that,
“the creation waits eagerly for the revealing of the sons of God. For the creation was subjected to futility, not willingly, but because of Him who subjected it, in hope that the creation itself also will be set free from its slavery to corruption” (Rom 8:19-21).
We see that in the resurrection there is the restoration of the old. This is why Paul, in 1 Corinthians 15, provides a long and detailed argument regarding the nature of our resurrection bodies and contends that it is our present bodies that will be resurrected and become glorious bodies (cf 1 Cor 15:42-44).
It is important then to grasp that the kingdom of God is not radically divorced from this world. After all, it is this world, and the people in it, that God desires to redeem.
This is why it is helpful to conceive of the kingdom of God in terms of the reign of God.
If we think of it solely as the “kingdom of God” we might be led to regard it only in terms of a place. Instead, the kingdom of God is wherever God reigns.
 Mark Allan Powell, Fortress Introduction to the Gospels, Kindle Version, loc 1540-48.
 This is why I argue in my book Understanding the New Testament and the End Times that the end-times have begun! The coming of Christ and the bringing in of the kingdom of God is the beginning of the last days.
 Much could be said here. For now, I will simply note that claims made by scientists that God cannot brought into a laboratory are true. Science, in and of itself, cannot make any assertions about God. God is beyond science. Consequently, the claim that science has proven God does not exist simply cannot be asserted. Science cannot make any assertions about God. They cannot conclude that something did or did not happen as a result of the work of God.
 This is despite the clear Christian teaching that God is ominpresent! Psalm 139:7-10 “Where can I go from Your Spirit? Or where can I flee from Your presence? If I ascend to heaven, You are there; if I make my bed in Sheol, behold, You are there. if I take the wings of the dawn, if I dwell in the remotest part of the sea, even there Your hand will lead me, and Your right hand will lay hold of me.”
 Note: when Paul says they become a “spiritual body” (1 Cor 15:44), he does not mean that it is no longer physical. After all, the tenor of the argument in 1 Corinthians 15 is that our bodies will become like Christ’s, which was certainly raised physically, and that it is our present body that is raised (note the word “it” in our English Bibles, 1 Cor 15:42-44, indicates continuity between the body that died and the body that is raised).