The founder of the Faceless Men came to believe that all of the diverse slave population of Valyria prayed for deliverance to the same god of death, just in different incarnations. Thus, in Qohor, the Many-Faced God is called the Black Goat; in Yi Ti, the Lion of Night; and in the Faith of the Seven, the Stranger.
This belief of a single god with many incarnations or "faces" came to be reflected in the Guild's temple, the House of Black and White, which contains a public shrine with thirty idols of death gods from across Essos and Westeros.
The worshippers of the Many-Faced God believe that death is a merciful end to suffering. For a price, the Guild will grant the "gift" of death to anyone in the world, considering the assassination a sacrament to their god. In the Guild's temple, those who seek an end to their own suffering may drink from a black cup of poisoned water, which grants a painless death.
As the Faceless Men forsake their identities for the service of the Many-Faced God, they only assassinate targets they have been hired to kill, and may not choose who is worthy of the "gift" by themselves.
He has faces beyond count, little one, as many faces as there are stars in the sky. In Braavos, men worship as they will... but at the end of every road stands Him of Many Faces, waiting. He will be there for you one day, do not fear. You need not rush to his embrace.
Death is not the worst thing. It is His gift to us, an end to want and pain. On the day that we are born the Many-Faced God sends each of us a dark angel to walk through life beside us. When our sins and our sufferings grow too great to be borne, the angel takes us by the hand to lead us to the nightlands, where the stars burn ever bright. Those who come to drink from the black cup are looking for their angels.
Kindly man: Our forebears came from half a hundred lands to this place of refuge, to escape the dragonlords who had enslaved them. Half a hundred gods came with them, but there is one god all of them shared in common.
Arya: Him of Many Faces.Arya: No. All men must die.
Kindly man: And many names. In Qohor he is the Black Goat, in Yi Ti the Lion of Night, in Westeros the Stranger. All men must bow to him in the end, no matter if they worship the Seven or the Lord of Light, the Moon Mother or the Drowned God or the Great Shepherd. All mankind belongs to him... else somewhere in the world would be a folk who lived forever. Do you know of any folk who live forever?