Isaiah begins with the heavens and the earth testifying to the fact the YHWH's people don't know Him [1.3]. This is not a simple "knowing" as in how you know that 2+2=4. This is an experiential knowing. This the "Adam knew Eve" kind of knowing from Gen 4.1 [KJV, from the Hebrew yada]. This theme of "knowing" can be traced throughout the whole of Isaiah.
When YHWH speaks in Isaiah 45.5-6, there is a beautiful structure that accents the significance of this knowing.
This does not merely show the importance of this intimate kind of knowledge, but this text proves, by its structure, that this knowing is rooted in YHWH's character and exclusivity.
Many who write theology books begin with proving the existence of God, the authority of the Scriptures, or some other foundational building block. But our good, dead buddy John Calvin began his theological opus ["The Institutes of Christian Religion"] with a chapter on knowing God. I think there is a beautiful distinction there. He began with relationship.
John Frame and Esther Meek would agree with him. Calvin said, "They that know God and know themselves cannot be proud." May it be so with us also as we pursue the kind of knowing that Israel seemed to so often lack.