This grand instruction was apparently on display at the entrance to the temple of Apollo at Delphi; it is also a very important part of what passes for contemporary wisdom. Be acquainted with yourself. Know who you are.
There is wisdom in this little catch-phrase. Self-knowledge is vitally important. Problem is, according to at least one strand of the western philosophical tradition, it is also impossible. I have in mind mainly Hume (who denied being aware of any self - just bundles of perceptions), Kant (whose doctrine of the transcendental ego cannot be delved into here, but involves the implicit barring of self-knowledge) and Wittgenstein (who points out that the self is that through which we view other things, and as such cannot be viewed in itself). The basic point is: because I am always the subject of my perceptions, I cannot also be the object. Even in introspection, I am still the one doing the looking, and so it cannot be I that I am looking at - or at least, not entirely. It's like trying to look at your own eyes - and without the benefit of a mirror.
We know this in experience - often others know us better than we know ourselves, and we are sometimes shocked when a close friend has to tell us an uncomfortable truth about what we are really like. Problem is, other people don't see us completely; they see only the externals, only our actions and the thoughts we choose to put into words. There are depths they do not see.
Is knowledge of God a pre-condition of knowledge of self? God knows me - really knows me - in a way that I cannot know myself and no-one else can know me. His knowledge of me is exhaustive and unbiased. I am object to him in a way I can never quite be to myself. So only he can really tell me what I am like.
If I am to know myself, that knowledge will come from above, not from within. But do I want to know what he might say about me?