You receive your powers from a magical source. These abilities might come from a mystical artifact bestowed upon you, the mastery of numerous magical spells, or pacts made with powerful dimensional entities.
Magic Origin heroes are occasionally problematic at Saint Joseph School.
The character has adopted or has been raised with beliefs from which has flowed a wellspring of power. This can be a belief in God, voodoo, or a pantheon of ancient or tribal gods. The powers will not go away unless the character loses their faith: and in most belief systems, the character will feel blessed, chosen by the object of their belief to be its instrument. Such faith is hard to shake.
Example: Shaman (Marvel Comics).
The character is actually a magical being. Whether from a world of magic, a lycanthrope or a summoned creature, this character toes the line between the magic and natural origins. The character may be a stranger to our world, created for some forgotten purpose, or simply trapped by circumstance.
Example: Goliath (from the TV Show Gargoyles).
The character has uncovered information that the character cannot forget and has forever altered their abilities. This might be ancient secrets from a musty tome or dark knowledge from a mysterious benefactor. The character might research further, but the initial knowledge that granted the character’s powers will not ever go away.
Example: John Constantine (DC Comics).
The character has a purpose, directed by otherworldly forces. Perhaps a family tradition, a magical artifact that was fated to come into the character’s possession, or the character was chosen by the previous child of destiny. The character has no choice in the matter: once providence has made its choice, there is no way to undo it.
Example: Devil Hunter Yohko (Anime).
The character is irrevocably bound to a source of power. This could be an object that fuses with the character or a spirit that possesses or directs them. The bond cannot be broken by any known means, and the character must learn how to survive in their altered state.
Example: Ghost Rider (Marvel Comics).
A character who must learn and re-learn spells to be effective is not appropriate. Any character who can choose to return to normalcy is not usually a good fit with SJS. This is fixed pretty easily by permanently burning the spells into the character's brain, unable to be forgotten (see Once Learned, it Cannot be Unlearned above).
Simply having a magical sword is not enough, and a concept that revolves exclusively around an artifact is unacceptable, be it a weapon, an amulet, or a set of bracelets. Any of these objects can just be thrown into a closet. The easiest fix for a concept like this is to somehow bind the character to the item (see Child of Destiny or Bonded by Magic, above).