When words are all you have, speak them from the heart, and your god will listen. Mystics, clerics, priests, druids, and any other servant of the gods can pray to their deity for power, their chanted prayers and devotion mixed with their own Mana creating a magical effect. Arcanists can, with a great deal of practice, learn to use prayer as a channel for their own magic, but those who choose to do so are few and far between; the practice is almost exclusively the domain of Mystics.
Though it is the most common method, prayer is not the only means by which Mystics can perform magic; frequently it is used to reinforce another Magical Tradition, such as a Jadefang Shantymage weaving prayers to their deity into their shanties, or an Iskaldur Runavejar making a short prayer to their deity in between each rune they place in a ritual circle.