The ninth chapter announces itself as the most secret knowledge, the sovereign science, directly perceptible, in accordance with right order, easy to practise and imperishable. That combination is unusual: secret and easy at once, and the chapter earns it by making a very large claim in very plain terms.
The claim is that the whole world is pervaded by Krishna in unmanifest form, that all beings rest in him, and then, in the same breath, that he is not in them, and indeed that beings do not rest in him either. Commentators call this the divine mystery, and no school of Vedanta could avoid taking a position on it. If beings rest in God without God being contained in or exhausted by them, then the relation is neither identity nor separation, and the whole of the later argument between the non-dualist, qualified non-dualist and dualist readings can be traced to how each handles verses like these.
The chapter also supplies the image of the great wind which, though it goes everywhere, always rests in space. Whatever the relation is, it is one in which the greater term is not diminished by containing the lesser and is not confined by it.
At the level of practice the consequences are immediate. Krishna is described as the sacrifice, the offering, the fire, the oblation and the one who receives it, which collapses the whole apparatus of ritual into a single presence. Those who worship other gods with faith do in fact worship him, the chapter says, though not in the proper manner, which is a sharper statement of the position chapter seven approached more gently.