Even though we as Christians want to counter Sartre, he is on to something. Brunner states that creatureliness bring with it a "sense of weakness, of transience, of nothingness" (pg. 53) that we want to reject. We want to be strong, fixed, and something, yet our creatureliness shows that we are not. He continues to say, "This knowledge of, and this recognition of, our creatureliness is not something we can take for granted." (pg. 54) Once we take our creatureliness for granted, we lose how we are. We are not God, yet, we desperately want to be. (Thus we take our creatureliness for granted.)
He continues by saying that "The more man is able to distinguish himself from the rest of creation, the more he becomes conscious of himself as subject as an 'I', to whom the whole world is Object, the more does he tend to confuse himself with God, to confuse his spirit with the Spirit of God." (pg. 54) And the more he confuses himself with God, the more he desires to be God. When we see ourselves as at the top of the Great Chain of Being (http://en.wikipedia.org/wiki/Great_chain_of_being), we start to try to continue climbing the ladder until we are at the very top.
We may be tempted to say that we do not desire to be God. Yet, "only the man who has not yet become aware of his nature as a spiritual being, who still regards himself as one object among many others, who thinks it is easy to reject the temptation to become like God." (pg. 54) If we do not take our creatureliness seriously, not only do we not want to be God, but we do not want to become like God. We would rather be impersonal objects than personal subjects.
Lastly on this subject, Brunner cautions us that "where man thinks God by his own efforts and does not meet the God who reals Himself, in the last resort cannot help" to have this desire to be God. (pg. 54) If we only think about God, we not only limit God, but we become God. If we do not encounter God, we only encounter ourselves. "It is only the encounter with the Living God which eliminates this error." (pg. 54) On top of this, it is only as a creature that we can met the Living God. Otherwise, we desire not only to be God, but to kill him.