Faith is a Stat in Dark Souls
Faith governs the Miracle Power and also increases the Magic Defense. It also controls the player's attack power for weapons that scale with Faith.
Miracles require a minimum Faith level to cast.
(You can acquire without the required stats and even attune the spell, but attempting to cast will leave your character scratching their head, literally.)
NOTE: Some weapons (e.g. Astora's Straight Sword) require a minimum Faith level to be wielded properly.
If you do not meet the minimum Faith required, you will stagger after every hit on an opponent.