The magic of the Old Religion is the form of magic used by most human sorcerers. It is the magic of the earth itself, the essence which binds all things together, and maintains the balance of the world.
Rules and Beliefs
The magic of the Old Religion is believed to be the essence of the earth itself. It binds all things together and is part of the very fabric of the world, which gives it the power to make solid what is in the mind, and un-solid what is already solid (Le Morte d'Arthur, The Wicked Day, The Diamond of the Day).
Because of this, the Old Religion is not considered to be a simple belief or worship, but an omnipresent force that holds everything in balance. It is unknown whether this force is considered to be animate or inanimate. Nimueh told Merlin that the Old Religion did not care who lived and who died as long as the balance of the world was restored, but it's unclear whether she meant that it was animate force with a will of its own that would choose whatever sacrifice it wished, or an inanimate force that could be directed to choose whatever sacrifice she wished (Le Morte d'Arthur).
Balance is a key tenet of the magic of the Old Religion. In order for something to be given, something must be taken in return. This principle is especially important to rituals relating to the power over life and death and blood sacrifices, both of which require a life to be taken (Le Morte d'Arthur, The Darkest Hour).Though not nearly as drastic, the principle of balance can also be seen in the casting of spells. When a sorcerer uses magic, the strength and amount of magic that they used often appears to correspond to the amount of energy that they expend. For example, at the end of Merlin and Morgana's magical duel, Merlin summoned a powerful whirlwind that threw Morgana through the air. The effort of casting such a powerful spell took a toll on the warlock, who immediately collapsed from exhaustion (A Servant of Two Masters).
Methods of Use
The magic of the Old Religion is practiced by most human sorcerers. The more contact they have with the power of the Old Religion, they more they can achieve with its magic. As such, creatures of the Old Religion (such as Merlin) are known to be incredibly powerful, as its magic lies within them and is the very source of their power. Those who serve the Old Religion (such as High Priestesses) also have access to this power (Le Morte d'Arthur).
Unless a sorcerer is highly skilled (or their emotions are heightened), they typically channel their powers with spells, rituals, and other instruments of magic. Most beginning sorcerers are also limited in this regard. Morgana, for example, who had recently learned to control her magic but not how to use it, had to rely on a poppet as a central focus for her spells when she used a Phoenix Eye to try to kill Arthur (The Eye of the Phoenix).
Some creatures of the Old Religion (such as Sidhe, Trolls, and Dragonlords) have their own unique forms of magic. As creatures of magic, it is likely that the Old Religion is the source of their power.
Followers of the Old Religion can also access its magic by praying. For example, when Sefa went to meet her father in the woods outside Camelot, she found Ruadan kneeling in an abandoned fortress (possibly a place of worship) and reciting a prayer:
"Crungon walo wide, cwoman woldagas, swylt eall fornom secgrofra wera, wurdon hyra wigsteal westen stapolas, brosnade burgsteall. Betend crungon hergas to hrusan. Forpon pas hofu dreorgiad, ond paes teaforgeapa tigelum sceaded. Crungon walo wide, cwoman woldagas ond paes teaforgeapa tigelum sceaded hrostbeages hrof..."
It is possible that this prayer is a call for victory over the enemies of the Old Religion. After Ruadan was killed by the Knights of Camelot, Gaius found such a call written on a piece of parchment. It was hidden inside an amulet that Ruadan wore around his neck (Arthur's Bane).