The first and last sections of Psalm 27 reflect David's contradictory stance towards God.
On the one hand, he says "...my light, my salvation...;" on the other hand he begs, "Hide not Your face from me, Do not withhold Your help in anger..." How can the psalm posit such inconsistent feelings by a man towards his God?
The answer is clear. Poetry can be defined as the ability to transmit to paper that which one feels at any given moment. The beauty of the Psalms lies within the poetic medium through which King David allows us to experience his thought processes. This permits us to feel the emotions which reverberated in his soul.
Man, by his very nature, is inconsistent. Perfection, absolute consistency, is left for God and His angels. The tension which exists between the sublime faith in God as one's salvation versus the harsh reality of life, where one might sometimes feel abandoned by God, leads to a complex set of emotions regarding faith in religion.
The drastic change in tone between the sections of this psalm has led some scholars to suggest that the two halves of this psalm were originally two separate songs put together by an editor. According to our understanding of this psalm, this is far from the truth. It is precisely this tension which King David intended to convey through the different tones of this psalm. Precisely when King David feels implicit trust that God will ultimately save him, he looks around in this world and sees his enemies, his misfortune and his uncertainty, all of which lead him close to despair.
Often our relationship with God wavers between blind faith and the practicality of a troubling present. The struggle lies in trying to overcome our fear of reality and simultaneously bringing our faith to the forefront of our lives.