Faith is a gift in every sense of the word. In a way, it is the quintessential gift of all the gifts given to the sentient races of Elwarne: it comes from the gods, it exists to assist in the relationship between races; it is a blessing both when utilised and when taught; and it contributes to the growth and prosperity of all sentient races.
Faith is the gift that allows a sentient being to have a close relationship with one or more of the gods (or other beings) of Elwarne. Faith is not simply belief in the existence of the gods – belief is fairly universally attested to (almost all races still have survivors from the original creation, and anyone who has met someone with the gift of faith has probably seen an answered prayer). Faith is relating to one of the gods, either by way of trust and covenant (the good gods) or by service and bargaining (the evil gods). There is actually an ongoing theological debate as to whether the original gift of faith is actually used to commune with the evil gods at all.
At any rate, the gift of faith allows sentient beings to become clerics – servants of the gods capable of doing their will on Elwarne. Combined with the gift of martial arts, the class of paladin is available to a sentient being.
Unicorns are the holders of the gift of faith. Their special relationship with the gods allows them to worship all of the good gods in unison, providing them all with clerical powers, and indeed with all the clerical powers of all the worshippers of the good gods.