A soul in peace is the true kingdom of God, among other things, because it recognizes but one authority. Its eye is "single;" looking in one direction, and having knowledge of but one master. It feels the deep import of the Savior's words, "Ye cannot serve God and Mammon." And while it recognizes but one authority, in distinction from a two-fold or divided authority over it, it cheerfully submits to that authority and harmonizes with it. It thinks what God thinks, desires what God desires, wills what God wills.
On the other hand, a soul not at peace is one which is rebellious against its rightful master, or which wickedly proposes to serve two rival masters at the same time.
Again, a soul in peace is the kingdom of God, because God rules in it and over it. It is true, his government is sustained, not so much by positive and outward enactments, as by the perfect adjustment of affectional and moral relations. But still it is a true government, although carried on less by force than by the truth mutually communicated and received, and by love harmonizing with love. In the truly peaceful soul, the life of God, including that which is perceptive as well as that which is affectional, seems to be reflected in the life of the creature. God is not more a living speaker to the soul than the soul, in a state of peace, is a living auditor. Moment by moment he communicates his will inwardly by a spiritual operation; and the intimations of his will are obeyed, by the soul which receives them, in the very moment of their communication. And this divine obedience is the obedience of harmony rather than of compulsion; the obedience of a subordinate nature yielding to and mingling with a higher and originative nature, through the influence of that beautiful attraction which always exists between kindred natures; but it is still that true and perfect obedience which God approves.
— edited from A Treatise on Divine Union (1851) Part 8, Chapter 12 by Thomas Cogswell Upham. His blog is managed by Craig L Adams and can be found here: http://thomascupham.blogspot.com