Now that I hear this thing I’ve been hearing all along, I’m actually a bit outraged. And amazed too. If we are not Him—and cannot understand Him—then how are we authorized by Him to step into His shoes? In order to understand Him?

And in my opinion it is heresy to use this expression—at least that’s my current view—because on the one hand we are commanded to know God, and how can He command us to do something we are incapable of doing? “Know the God of your father!”
I’d be glad to hear an explanation that makes sense!

If you want an explanation, then please make the effort to state your question briefly and clearly. I recommend leaving the outrage aside.

First, one must distinguish between “knowledge of God” and “knowing” God as though He fell under the category of a “thing” according to the concepts of human cognition, and were given as an (external) object of knowledge, like every other object in reality as it is presented in experience—that is, as a “thing,” one being among beings embodied within the framework of natural perception or in the revealed “world.” To properly grasp the correct meaning and deep intention expressed by the sages of Israel in this well-known saying, “Had I known Him, I would have been Him,” one must understand that “knowing God” is neither synonymous with nor equivalent to “knowledge of God.” The word da’at in biblical Hebrew does not mean “knowledge,” but rather, as explained in Hasidic teaching (the Chabad school), da’at means “felt awareness and attachment,” or in another formulation, “the bonding of spirit with spirit”—that is, a deep, direct, root-level connection between things from the innermost core of their essential being, bringing them into cleaving and union with one another. Alternatively, this essential connection is emanated and drawn directly from the source of primordial unity, in the spirit of “two are better than one.” Thus da’at points to the mystery of “union” in Kabbalah, which is the state and condition of unity (covenant) between male and female, who constitute two complementary aspects of one another (“God made one corresponding to the other”), or opposite yet complementary faces of the same all-inclusive spiritual essence called “man,” as it says in Genesis: “male and female He created them … and He called their name Adam.” Therefore, when it says, “And the man knew Eve his wife,” it means that the two were included in one another and became one essential entity, in the aspect of “one flesh” (“flesh” being the opposite of “fracture,” indicating the idea of repair that joins and unifies two opposites or halves of one whole being).

Therefore, knowledge of God means the state of true cleaving to the source of eternal life, blessed be He, in the spirit of “and you who cleave to the Lord…” according to the secret of the pure intention of union. (In Kabbalah, intention for its own sake points to the “middle line” among the three lines of the repair of the world, through which the ten sefirot of the Tree of Life are extended, and da’at is fixed in the middle line.) This straight intention for its own sake (“Yisra-el,” the straight to God) is the very essence of da’at in its complete repair, as it eternally exists and stands in its primordial source and root, where sin has no place in it (sin meaning a missing of the intention of union). In other words, da’at is the foundation of intention for its own sake—for the sake of complete union and joining face to face.

By contrast, “knowing” God is wholly impossible, because the Holy One, blessed be He, is not, in principle, the sort of thing that can be given as an “external” object to our cognition, perception, or comprehension—including abstract intellectual comprehension and mental representation—for “no thought can grasp Him at all” (brought in the essay “Patach Eliyahu” in Tikkunei HaZohar), and as is written in Maimonides’ principles: “the apprehensions of the bodily cannot grasp Him.” Therefore it necessarily follows that only the Holy One, blessed be He, alone knows and is capable of knowing Himself in a complete and absolute manner—and this must be so not by means of a knowledge dependent on an object set over against it, to be grasped, as it were, as the content of that knowledge. From this it follows that the object of knowledge is itself the necessary condition for the possibility of such knowledge. But with the Holy One, blessed be He, there is no distinction between the knowledge, the object of knowledge, and the subject who knows or bears the knowledge, since these three aspects or faces—distinct but not separate from one another—the knower, the knowledge, and the known (Maimonides’ version of the Aristotelian triad: the intellect, the intelligent subject, and the intelligible object) are one in essence. Therefore they are not separately discerned or distinguished from one another at all. That is, God’s so-called knowledge of Himself is simply inherent within Him and is not something added to His existence; rather, it and the Holy One, blessed be He, are truly one. Hence this “knowledge” is utterly intrinsic and exists by virtue of the very essence and being of God alone (even though there is truly nothing apart from Him). No one else can know Him in the truth of His complete and absolute essence, in the original essence of His being, for He is perfectly whole, singular, and unique.

Therefore, “had I known Him” means “I would have been Him,” and “had I been Him,” then I would not have known Him at all as an object of that knowledge. That is to say, God’s self-knowledge is only a borrowed term, but it is not in the category or aspect of “knowledge” at all according to our concepts, since it is beyond all comparison and relation, and is not similar in any way, neither in kind nor even partially, to human knowledge, which is inherently deficient and object-dependent, and divided into three aspects, as stated above. And that is precisely what the prophetic words mean: “My thoughts are not your thoughts…” God is not to be known, but to be in da’at with. Cleaving does not mean knowledge of God; on the contrary, true cleaving is “until one does not know” (also related to the term found in kabbalistic thought: “the head that is not known at all”), and this is also called “know—that one does not know,” because the ultimate end of knowledge is specifically not to know. Its meaning is to be in a state and condition of non-knowing in the mode of “until”—that is, forever, and indeed until the concealment of the very end of knowledge and its nullification in essence (also called “the concealment of the essential nothingness” in Hasidic thought). And this is precisely the main essence of faith for its own sake, which is cleaving and true da’at (met, “dead,” with the same letters as tam, “simple/whole”: that is, simplicity of da’at is acquired in faith, which is the nullification of knowledge in the aspect of “not knowing,” the very opposite of the nature of knowledge; and thus a person is as dead and whole, that is, complete toward the Aleph, which points to the Master of the world, the one known to him and his primordial source). Maimonides explains these matters at length in his penetrating analysis, in his philosophical teaching, which he presents systematically in his book Guide for the Perplexed, as well as in the Laws of the Foundations of the Torah, in the first “Book of Knowledge” section of his Mishneh Torah, the Yad HaHazakah. See there.