WARNING! WARNING! WARNING! This data is old, no ancient. It's here purely for reference!
The faith rods, maces, and flails are carriers for allowing a player to request their god to perform a service for them, quickly and without the guild point cost (beyond the 40 guild points to hold the item). The services that each item allows the player to request depends on which powers have been infused into them. Note that this system means that wizards, witches and any others not worshipping a god are unable to access the powers contained within the items. The ability to hold the items are based on skill in faith.items.rod - the more powers they have, the higher bonus that is needed to hold each item. Since Gods are, of course, fickle beings, having enough skill to hold a faith item one moment does not necessarily mean you have the ability to hold it on each and every attempt. However, it is possible to pass a non-failure threshold for each item, where the God will always be satisfied with your devoutness.
Each item contains a reservoir of power that is used to contact the God and request that they perform one of the rituals allowable by the item. The items have differing amounts of total charge that is available to them, some using up the pool of charges and crumbling to dust far quicker than others. The items give off a glow which indicates how much charge they currently contain. The glow system is one of absolutes, rather than one of relative measures. This means that while some items will start off with a higher charge glow, if two items give off the same glow, then they have roughly the same charge remaining. The glow is indicated in the description of the item.
"A brilliant glow along its length reveals ..."
As well as their powers as faith items, the rods, maces and flails also function as blunt weapons. Though they are not particularly impressive weapons - being, on the whole, rather too difficult to use relative to the damage they are able to inflict - they can be handy in a tight situation.
The powers available to the items are all existing rituals: detect alignment, stasis, cure medium wounds, bless, dust devil, minor shield and fear. The strength and duration (e.g. size of dust devil, duration of bless, etc) of the rituals cast from the items is not the same for all users, it depends on skill in faith.item.rods of the person using the item. Some rituals, like dust devil, are capped and will give significantly lower results than the actual rituals. (The biggest dust devil possible from any rods is Large).
Some player shops will provide a selection of the faith items - though you will of course have to pay more than they are worth in the typical general stores. They are also available first-hand off priest NPCs, some of which are listed below. Typically, it might be more advisable to obtain some of these items by using fumble, or asking a thief for assistance - killing some of the priests, in particular those who provide a valuable service, is often frowned upon. Silver flails are available solely from Althea of Pishe, who is typically found in the Temple of Small Gods. Yellow rods are available from Kess of Sek, typically found wandering around in the temple complex on and near the Street of Small Gods. The red maces can be obtained from the different named NPCs in the east wing of the TOSG. Forest green canes are available from Dickens (In the Temple of Small Gods) and Hermal, in the Sandelfonian temple. Blue maces can also be found on the named NPCs in the east wing of TOSG or on Harry, who also has a part-time business of returning people from the dead, so please do NOT kill him. The rest, as well as some of the above, are available from priests wandering around Ankh-Morpork, often around the Street of Small Gods.