So, the "kingdom of the heavens" can refer to the spiritual and manifesting Presence in all the cosmos. Since the Divine is Infinite, we can assume the possibility of the infinity of the cosmos when we use cosmos for all Reality, all dimensions.
Going back to Main referring to the potential of life within us, then, that links to spiritual formation in this cosmos. The life that permeates all Reality is the life that constitutes our very being. The same life flowing of Life in all dimensions seeks to discover and manifest itself in the unique configuration of "you" and "me." This life, being God, or Life, has mind, has intelligence, and manifests through our brain and nervous system. Imagine! your little brain is the vehicle of the Mind of God.
This "kingdom of the heavens," then, refers to the Presence of Love everywhere, even in the "where-ness unseen" but present. In and by this Presence we Practice, and in such Practice, with its resultant transcendence, we manifest godlikeness. We can, in the words of Meister Eckhart, "take God with [us], in all things." Note Eckhart on this manifestation of transcendence in the kingdom, which is not elitist, not other-worldly, but more the natural for being practical and beautiful, as well as this-life-affirming.
The man who has submitted his will and purposes entirely to God, carries God with him in all his works and in all circumstances. Therein can no man hinder him, for he neither aims at nor enjoys anything else, save God [thus, all affection is for being in God, not in the object itself]. God is united with Him in all his purposes and designs. Even as no manifoldness can dissipate God, so nothing can dissipate such a man, or destroy his unity. Man, therefore, should take God with him in all things; God should be always present to his mind and will and affections. The same disposition that thou hast in church or in thy cell, thou shouldst keep and maintain in a crowd, and amid the unrest and manifoldness of the world.
Eckhart speaks of the transcendence Main refers to. This transcending is not of space or time but within space and time, a transcendence of consciousness that makes a person not less natural but more natural. Much of the evils of religions, and societies, would have been avoided had this been understood and practiced by leaders of religion, for they are primarily at fault for prejudicial injustices of adherents.
Even the wonderful, liberating, and holistic message of Jesus was divided and compromised into a divided cosmos. This cosmos was marked off by the dimensions of our single experience of the totality of God. Persons were left to long for heaven in a someday, far-away place. Heaven has been knocking at heart-doors for eons, while most of Christianity is incarcerated in a divided mentality, rather than "seeing" the kingdom of the heavens, even Heaven, alive, here, now, within and all around ~ in the seen equally as in the unseen. Certainly, this Heavens is not limited by your eyes!
The kingdom refers both to Immensity and Specificity. The Divine is as much in a grain of sand as in a galaxy. God is as much in you as in the entire cosmos. God is wholly the Potency of All and seeks to manifest that same All-Potency in the specific I-Am-you-are and the circumstance of your life. The potencies of All comes in This. Reminding ourselves of opening words from St. Teresa of Avila, the One of the heavens can be met and loved, met by and loved by, in the little heaven of their souls. And this meeting and loving is the same as being met and loved, for in God the one is the same, for both are in the One. Both are, which is to say is, the One.
At last, you might say, "But how do I experience this growth in the Kingdom of the Heavens?" The word experience is key. We United Methodists stress, as did John Wesley, experience as central to living in Christ. Yet, this experience in Wesley was a logical and objective experience. This does not mean such experience must be devoid of subjective feelings.