Negative theology is a theology in which God is said to only be known through attributes that separate him from this world. Per Act and Being:

It is a dogma whose truth is almost everywhere stated or assumed that the human being cannot know the essence or being of God. One consequence of this for our language is that, as they stand, out words are simply incapable of speaking of the creator. That is the truth underlying what is known as the negative theology: that God can best be characterized by thinking away the limitation inherent in words designed – or so the theory goes – to speak of created things.

Negative attributes would include: Simple, perfect, eternal (in the timeless sense), omnipresent, omnipotent, omniscient, immutable, impeccable, ineffable, invisible, infinite, unknowable. They are negative because they distance God from creation. They explain not who he is, but what he is not.

Atheist George H Smith, predictably is frustrated by negative theology. He first explains what it is and then explains why it is wholly unhelpful in understanding God:

Some of God’s attributes are obviously negative: “immutable” tells us that God does not change; “ineffable” tells us that God cannot be described; “infinite” tells us that God is not finite; “invisible” tells us that God is not visible. Even some terms that appear to be positive are essentially negative. To say that God is “eternal,” for instance, is to say that God is “not subject to temporal succession.” Furthermore, attributes such as “omnipotence” and “omniscience” signify capacities without limits, so they also stem (at least partially) from the negative way.

While the negative way logically presupposes positive knowledge of God’s nature, most of God’s negative qualities, because they entail the inherent contradiction of the “unlimited attribute,”cannot be translated into positive terms. The negative attributes of God do not provide us with any real knowledge of God’s nature; they are mere pseudo-attributes. Implicit within these characteristics is the premise: “reason will never understand this.”…

If the Christian wishes to use positive characteristics for God while retaining their meaning, he must reduce his God to a manlike or anthropomorphic level. On the other hand, if these predicates do not mean the same when applied to God as they do when applied to natural entities, then they assume some unknown, mysterious meaning and are virtually emptied of their significance.

A negative theology, a theology that distances God from his creation is one in which God cannot be known in any real sense of the word. Christianity, on the other hand, which teaches that Christ was the image of God, holds that God is knowable on a personal level.