What is the use of faith if God doesn't need/require anything from humans to save them? According to soteriologies (e.g. Calvinism) that believe God makes the choice to save people without expecting any decision from them first, what is the role of faith? If the decision making process is not mans' in the first place, why is it that God says people will only be saved by faith? What does their faith accomplish?

Put another way, why does he require obedience if there is no choice in the matter anyway?
There are a few facets to this, but the Calvinist perspective (though not nec. only the Calvinist perspective) would say that faith is the mechanism for salvation, because it gives God the glory; it doesn't give man room to boast (also, it doesn't help that man doesn't quite cut it with his works).

The apostle Paul addresses this in his letter to the Ephesians:

Ephesians 2:1-10 (ESV) [Emphasis mine]


  1 And you were dead in the trespasses and sins 2 in which you once
  walked, following the course of this world, following the prince of
  the power of the air, the spirit that is now at work in the sons of
  disobedience— 3 among whom we all once lived in the passions of our
  flesh, carrying out the desires of the body and the mind, and were
  by nature children of wrath, like the rest of mankind. 4 But
  God, being rich in mercy, because of the great love with which he
  loved us, 5 even when we were dead in our trespasses, made us alive
  together with Christ—by grace you have been saved— 6 and raised us up
  with him and seated us with him in the heavenly places in Christ
  Jesus, 7 so that in the coming ages he might show the immeasurable
  riches of his grace in kindness toward us in Christ Jesus. 8 For by
  grace you have been saved through faith. And this is not your own
  doing; it is the gift of God, 9 not a result of works, so that no one
  may boast. 10 For we are his workmanship, created in Christ Jesus
  for good works, which God prepared beforehand, that we should walk in
  them.


Reformed Theology understands the order of salvation as beginning with regeneration (being "born again" -- see John 3) with a capacity for faith.   This faith is not something that is effected through the will of sinful man (who is "dead in [his]...sins"), but rather, it is a gift from God.