According to the Rambam, our highest human potential is achieved when we use our minds creatively to know G-d. And indeed the Rambam holds that the first and second positive Mitzvos are to know (note: not "believe" but know) that there is G-d and that He is One. Further, who is there among us that does not yearn for this very thing, to feel G-d's Presence in this world and to know Him.
Anyone embarking upon the fulfillment of these Mitzvos and the attainment of their potential is, however, immediately struck by the following paradox, as stated in our daily davening:
You were [the same] before the world was created; You are [the same] since the world was created.
And indeed, G-d answers:
I, G-d, have not changed.
And Moshe Rabbeinu reinforces this (in Devarim) with:
You have been shown to know, that Havayah is Elokim, there is none beside Him.
Yet, according to Torah, the world truly exists and was created ex nihilo, something from nothing (yesh m'ayin). How is this possible?
The Kabbalists state the problem even more exactly. They teach that the power to create ex nihilo is possible solely within the province or capabilities of the Ein Sof - that is, G-d in His Essence.
If you say that Ein Sof is only G-d as He is in His Essence, then you must say the creation of the world is from His Essence and that creation therefore has effected some change in Him, G-d forbid.
You may, in answer, posit that Ein Sof is a characteristic of an entity other than G-d in His Essence. But then you must are forced to conclude that exists an entity, other than G-d, that also has the quality of being Primary - that is, preceding all existence (because Ein Sof by definition is applicable only to an entity which precedes all others). But, of course, such a position is untenable.
[Ein Sof means "without limit." But if you have two entities, then by definition they are limited - they are, by definition, NOT the other entity. Therefore, they are not Ein Sof.]
How then are we to understand G-d and our existence?