Eko was an african bushman who was converted by missionaries. Many missionary groups that I am familiar with like to take leaders from the peoples they convert and after they have a basic understanding of english and doctrine, put them in a new suit and fly them back to the US to go to seminary and become native clergy.

When the plane crashed, Eko's faith was still present, as we can see by him seeing prayer as the most helpful thing he could do for the crash victims. But after the Others started taking survivors and he was forced to kill two of them in self-defense, I think he lost his faith. Just like how in Firefly Mal lost his faith at the end of the Battle of Serenity Valley. He then "went native" again and stopped speaking because of his own inner turmoil and perhaps the reality of what happened finally hitting him. Possibly he also stopped speaking because of resentment to the westerners; if he did not convert and enter this new life he would not be in his current situation.

He is unable to turn his back on his fellow survivors and becomes the warrior and tracker of the group. When he finds out that Michael left in a (hopeless) search for Walt and that Jin is leaving to search for Michael, that breaks the turmoil that Eko found himself in and he starts to become himself again, even standing up to Ana Lucia to save Sawyer's life.

Become a foil to Locke. They are the hunters/trackers for each group of survivors and each is a person who is a champion of faith. Locke's primitive faith in the island and Eko's newly regained and strengthened Christian faith. Perhaps Locke will become increasingly unhinged as he still sees himself as a "chosen one"(his mother implanting the belief that he is destined for greatness, his response to Desmond's question) and is unhappy with the explanations of the island phonemena being explained by the Dharma Initiative.