In saying that the soul is God’s kingdom, it should be kept in mind that the term KINGDOM is relative in its meaning. It implies the idea of a governor, as well as of that which is governed. Accordingly, it is not only the place where the king dwells, but the place of the king's authority. It is not only the king's home, which is the original meaning of the term, but the place which the king rules over.
In a certain sense God rules everywhere. There is no place where he does not dwell. Nor is there any place which excludes his authority; He rules in hell as well as in heaven. He rules also over all earthly things; over things material as well as immaterial. He rules over all moral beings. He rules over men.
Undoubtedly there is an universal kingdom; — a kingdom including all things. But, ordinarily, when we speak of God's kingdom on earth, we mean his spiritual kingdom, — the kingdom of mind, and not of matter; the kingdom of hearts, and not of outward forms and localities. The divine throne, erected everywhere, is especially and emphatically erected in man's spirit. The soul of man, a fit subject for the divine administration, always is, when renovated, and always ought to be, God's kingdom. Hence the remarkable expression of the Savior: "THE KINGDOM OF GOD IS WITHIN YOU."
But in speaking of the human soul as a fit subject for the divine administration, and in saying that it ought to be God's kingdom, we imply, that, under certain circumstances, by doing or being what it ought not to do or ought not to be, it is not God's kingdom. And thus we come to our proposition. It is the soul IN PEACE, (that peace which the Savior speaks of when he says, "Peace I leave with you, my peace I give unto you,") the soul in peace, and not under any other circumstances, which constitutes, in the truest and highest sense, the kingdom of God. "For thus saith the Lord God, the Holy One of Israel, In returning and rest shall ye be saved. In quietness and confidence, [that is to say, in the quietness and peace of faith,] shall be your strength." Isa. 30:15.
— edited from A Treatise on Divine Union (1851) Part 8, Chapter 12.