Holy symbols can be made by having an unblessed symbol blessed by Brother Jered in the monastery west of Edgeville, or by blessing it yourself by using a holy book or book of balance on an unblessed symbol at 50 Prayer, requiring a few prayer points. Blessing by Jered requires at least 31 Prayer. They have the third highest prayer bonus for an amulet in game (after the god stoles and Dragonbone necklace). They aren’t expensive/hard to obtain, therefore are commonly used in conjunction with Proselyte armour and other high prayer granting armour when using protect prayers as a form of defence when training.
A holy symbol is required for Animal Magnetism and The Great Brain Robbery.
You can make a holy symbol by using a silver bar on a furnace with a holy mould. You will get an unstrung symbol and can then use a ball of wool on it.
Wearing a holy symbol will decrease the chance of Vampyres healing when attacked (this healing is similar to Banshees).