Since there is nothing else but His Oneness, what room is left for us to make any difference?
The Short Answer:
Yes, there is nothing else but His Oneness, but that Oneness includes the fact that He created a world. A world means a place where there is free choice.
A Little Longer Answer:
This question touches the essence of Torah and Judaism: There is nothing else but Him, and we are serving Him. Yes, that is a paradox -- for if there is nothing else but Him, what are we doing here serving Him?
It happens in the most basic creed of Jewish faith. A Jew declares morning and night: "Hear O Israel! The Eternal is our G-d. The Eternal is One." If He is One, then there is nothing else. Nothing, not even this world is external to His Oneness. Everywhere you look, there is nothing but Him.
And then we say, "Blessed be the name of the glory of His kingdom, forever and ever." Now there is a kingdom! Something external to the king that he rules over! It implies that there is a world independent of Him, only that He dominates it. But didn't we just say that there is nothing but His Oneness?
The easy way out is to say that all this world and everything that occurs within it is no more than an illusion. Plenty of existential philosophers, including the Hindus and Buddhists, have come up with this exit door: We just think we exist. We think we have free choice. But there is no truth or meaning to any of this, and best we just shake ourselves out of the dream and wake up to the fact that we are no more than figments of our imagination. Some call it transcendence, others enlightenment. Free choice, to them, is just another delusion of those trapped in darkness.
But Torah does not provide any easy exits. It begins, "In the beginning, G-d created the heavens and the earth." He really did. And then He told us, Do this, don't do that. Which means He considers it a reality.
When Rabbi Menachem Mendel of Lubavitch (1789-1866) was asked for proof that our reality is not an illusion, he pointed to the Mishnah concerning Shabbat: One who pretends to harvest on Shabbat by illusion (sleight of the hand) is free from punishment. If all the world were an illusion, the rabbi explained, nobody could ever be culpable of breaking the Shabbat!
The Creator created a world. A real world. And that means a place where our actions make a difference. He decided this place would really exist -- it wouldn't just be an illusion of our minds. And so, that's what it is.
How this jives with His Absolute Oneness is a paradox. So He created a paradox. That's what creating a world out of nothing is all about -- doing the impossible. Creating a world means creating a second reality: The first reality is that there is nothing else but Him. The second reality is that there is a world here that He is sustaining. Both realities are true.
In Chabad Chassidut, these two realities are called Daat Elyon and Daat Tachton, literally, the Higher Consciousness and the Lower Consciousness. Answers 1-4 above were entirely from the perspective of the Lower Consciousness.
One clue to grasping this paradox is provided in the classic work of Rabbi Meir Gabbai, Avodat HaKodesh:
"Just as He has the power of infinitude, so He has the power of finitude. For if you will say that He does not have this power of finitude, you have thereby detracted from His perfection."
The capacity of this Oneness to express itself in finite forms and pluralities is then no more than the ultimate expression of perfect Oneness. Cheap Oneness is impotent. G-d's ultimate Oneness is expressed in His creating us with our capacity for free choice.
There is an analogy given to understand this: A wise and enlightened teacher has attained a great truth. But is he able to express this truth in such terms that a simple student can grasp it? Even if he will distinguish the elements of this truth that his student can relate to from those far beyond him, even if he will reduce the intensity of his revelation a hundred times over, the idea will still remain outside of the students world. To relate his experience to the student, he must dress it up within parables and examples borrowed from the students world, using terms, objects and characters entirely foreign -- even diametrically opposite to the essential thought he wishes to communicate.
If he cannot accomplish this task, then no matter how great and brilliant his revelation, it turns out to be limited in scope. It exists in his world, but not in the world of his students. If, on the other hand, he is successful, he will have revealed the true boundlessness of his truth, that, like the speed of light, it functions equally from within every frame of reference.
So too, above: If Oneness excludes the possibility of anything other than simple, infinite oneness, then that itself is a limitation. True Oneness is expressed in the capacity to bear its opposite: The autonomy of a created being. When these two poles co-exist, neither diminishing from the other in any way, Oneness is expressed in its ultimate form.
Like the movements of an oriental martial artist, fusing stillness and motion, gentleness and force, the Torah dances between the singularity of G-d and our multifarious reality. Grasp one of these truths alone, and you have no truth at all. Grasp both at once, and you have G-d.
For more on this topic, see The Bubble.
On to the next question:
If G-d is the Primal Cause, doesn't the buck stop there?
The Paradox of Free Choice - Six Questions
1) Determinism: Isnt everything predetermined by the mechanics of the universe?
2) Robotism: G-d knew exactly what I was going to do when He made me this way. Im just a programmed machine. How can I be blamed for being what I am?
3) Prescience: Since G-d knows the future, what choice do we have in it?
4) Omnipotence: G-d wants something to happen and it happens. So how could I possibly choose to do something He doesnt like? Whos more powerful, after all?
5) Oneness: Since there is nothing else but His Oneness, what room is left for us to make any difference?
6) Primal Cause: If G-d is the Primal Cause, doesnt the buck stop there?