“Old faith” is a euphemistic reference to the belief system of the Children of Darkness (or the Children of Satan). They credit Satan as the basis of their faith, convinced that they are doing his work against God on Earth. According to the old faith, the Children of Darkness are guided in their beliefs by the five Great Laws, which state that such vampires are evil and cannot enjoy any goodness in this world, such as a luxurious lifestyle or drinking the blood of good souls. The Children of Satan in Rome lose this faith sometime before the eighteenth century, shortly after their coven master (Santino) abandons the old faith for new beliefs. The Children of Darkness also lose this faith during the eighteenth century, when Lestat convinces their coven master (Armand) that the old faith is unreasonable during the Enlightenment.
References to the old faith can be found in The Queen of the Damned (1988), The Vampire Armand (1998), and Blood and Gold (2001). For more perspectives on this idiom, read the Alphabettery entries Armand, The Children of Darkness, Children of Satan, Great Laws, and Lestat de Lioncourt.