This is a central text of the Reconstructionist movement (founded by Kaplan), second in importance only to his "Judaism as a Civilization". The book enlarges on his idea of functional reinterpretation of Judaism (what he calls "revaluation") and applies it to the entire ritual cycle of the Jewish year, to bring it up to date and render it more appealing to contemporary Jews. I'm hardly qualified to comment on this broader objective but will instead concentrate on the implications of the title--the author's concept of God.
It is an oversimplification to understand Kaplan's God as the human instinct or energy driving truth, justice, morality and esthetics--essentially, all that is virtuous in life. But in my reading, that pretty much captures it and distinguishes this notion from the more traditional all-powerful omnipresent, omniscient divinity that actively intervenes in life by controlling behavior and metes out rewards and punishments. (I'm well aware that professional theologians might take issue with that assertion, and I welcome corrections.)
As Mel Scult states in the book's introduction, "When we speak of God, we speak metaphorically, but the metaphor nevertheless stands for something real ... God is neither a being nor a philosophical abstraction ..." Kaplan writes: God should be thought of as "divine qualities" in man, "the sum of everything that renders life significant and worthwhile." As such, God must be actively sought to achieve self-fulfillment. Self-fulfillment, according to Kaplan, is the modern equivalent of salvation, which in traditional Judaism is "having a share in the world to come". God is the power that makes for salvation. He goes on to add that God must be felt as a presence--that we are in God and God is in us--not understood as a mere idea, if one is to know and be guided by "him".
Kaplan places great emphasis on subordinating one's own desires to what is best for the greater community/society. He laments the current (1936) state of the world, citing poverty, disease, crime, ignorance, insecurity, fear and hate, all resulting in large measure from class distinction and monopoly. (At times, his theology sounds like liberal reformism.) Striving to improve conditions for the larger aggregate is part of the divinely inspired mission. It is not enough to live a personally virtuous life because man is not a self-sufficient individual being. He points out that the Jewish religion was the first to affirm and fortify morality, and, accordingly, to include social reform as an integral objective.
Near the end of the book, Kaplan declares that the God he espouses--what he claims to be the highest evolved concept and the one most compatible with contemporary life--must be consistent with rationality and scientific truths rather than conformity with historically revealed law. In today's world, Jewish religion must identify God as the spirit in human nature that urges men by means of their ethical insights to fulfill the destiny of the human race.