In Anima: Beyond Fantasy, magic is divided up into paths, some of which are opposed to each other. The rules quite clearly state that it costs twice as much to advance along a path opposed to one which a character has mastered… But they entirely neglect to define what constitutes mastery. (My suspicion is that something was lost in the game's translation to English.)
A mage's progression along a path is measured from 0-100, with a spell being learned every even-numbered level above zero. Presumably, therefore, path level 1 wouldn't consitute mastery in any sense.
However, I very much doubt that you have to reach level 100 to be a master, as the high end spells of the path ('Divine magic,' in game terms) are only usable by players with the aid of GM intervention – and the spells a little lower-level than that are 'High Magic,' which typically require characters to permanently give up their humanity before being castable. Since many of the powerful mages listed in the setting material do seem to still be human, presumably mastery does not require that level of path knowledge.
So, for rules purposes, at what point has a path of magic been mastered? I could easily houserule it, but if there's an official answer then I'd prefer to use that.