Do you want really want the main premise? Or do you mean the main point/conclusion?

He imagines what is/was on God's mind.

Yes, Ender. He wrote from the standpoint of the Spirit's consciousness. You may call it God's consciousness. You should understand the Hegelian idealism as a process in which the Spirit is trying to realize itself through thesis-antithesis clash, which of course produces an ephemeral synthesis. Synthesis, in time, becomes a thesis too. Remember that negation is a Hegelian principle that is destructive, yet, ironically, productive at the same time by virtue of providing the historical progression. He is the man who introduced the notion of historical progression to the Continental Philosopy, and we can say that there was no concept of History before Hegel. As opposed to particular behaviors, Hegel is interested in the modes of consciousness. Thus, he writes the philosophy of history taking the changes in human consciousness. He mentions Unhappy consciousness, Sick consciousness, Stoic consciousness, etc. as modes of behaviors that will ultimately realize themselves in the "Absolute Knowing," the end of the dialectic. Dialectic is probably the most important term in his philosophy. We can desribe it as a process of exchange (thesis-antithesis) in which the thesis negates and absorbs the properties of its antithesis... Also, his Master and Slave dialectic is terribly important. In this type of power struggle, the slave is dominated by his master and forced to work on nature. It is through this domination and labor that the slave is the one that confronts the radical indeterminacy of his own being and produces culture that may not occur to the master. Master's consciousness is a dead-end. Hegel is difficult to read. He is a Hawk. Be careful to read him in different periods of your life!