Faith is a term referred to a stat in Final Fantasy Tactics, which determines the effectiveness of spells casted by and to the player. A player with a high Faith casts stronger spells, receives more damage coming from magic source, and status spells will hit him more often. A player with no Faith is basically inept at magic combat, and it becomes quite hard to either heal, hurt or afflict it with magic spells. Jobs associated with high Faith includes Black Mage, Time Mage, White Mage, Calculator, Geomancer, Summoner, Oracle, among others.
Contrast to the Brave factor, which determines how much physical damage the player receives or inflicts.