Spells for Priests and Witch Hunters are very different from the Mage spells: first of all these classes learn their Prayers (so are called their spells) from their related Cult, a form of organized religion. The use of reagents is different too: the Prayers are divided into Basic Prayers, common to all the Priests and the Wich Hunters, and Cult Prayers, derived from belonging to a specific Cult that the Hero has to choose at the start of his career. Basic Prayers share all the same reagent, the Blessed Water, while each Cult has its own single reagent. Prayers are cast as Spells are cast: the Priest has to make an Intelligence Check, if it's failed the Prayers cant be cast but the Priest doesnt consume the reagent (except if specifically indicated by the prayer). The number of Prayers that an Hero has at the start of his career are indicated as following:

Priest: 2 Basic Prayers, 1 Cult Prayer, 4 Reagents
Witch Hunter: 1 Cult Prayer, 1 Reagent

The number of Reagents can be increased as the other stats and skills of the Hero. Learning new Prayers is allowed too, with the maximum number of prayers (basic and cult cumulated) of IN-2. The Witch Hunter cant learn basic new Basic Prayer, but can learn up to 2 new Cult Prayers.