“Once, everyone used to know magic", at least according to Ceria's grandfather. It was as simple as breathing even for children, and all half-elves learned Tier 2 magic before they’d reached their teenage years. But in Human cities and other nations, students study under older mages or travel to schools like Wistram Academy.
Magic surrounds everything, but an individual’s capacity for magic varies. And the same goes for the surroundings. Thus, in a highly magically charged environment spells are naturally easy to cast. In a null zone, a [Mage] must rely solely on their own internal magical energy to cast spells.
The potential of becoming a [Mage] differentiates itself from species to species. Half-Elves, for instance, have a greater affinity towards spellcasting than Humans.
Gazers, too, are a race naturally inclined towards magic, as they are highly magical beings like the half-Elves and to an extent, the Lizardfolk. However their magic is more chaotic than that of typical [Mages].
Minotaurs are one of the species that have the smallest affinities towards magic, making it even an achievement for them to cast only 2nd Tier spells.
That is not to say that those races have to abandon the magecraft because they think it won't add anything to their military prowess. One way to go around that problem is shamanism. A [Shaman], chosen by the tribe, will draw Mana not only from the environment but also from the mana supply of their tribe's member, making them in a large tribe especially fearsome. The downside of that method is that the [Shamans] create the spells out of a collective will and lack structure and form. 
Magic Test Edit
There are several ways of testing the affinity of a person towards magic. The traditional Human method is by saturating the area around the tester with raw Mana and allow them to demonstrate their magical ability—or lack thereof. If they possess the potentail, they would be able to cast spells by just imagining, be it fire, water, or a shiny new pot.
The Half-Elves draw a silver glowing symbol with their wand in the air. The testee is supposed to look at it and show if they can understand it. To non-mages it looks like a bunch of squiggles and straight lines put together randomly.
Miracles are thought by some to be an older, extinct form of magic. Instead of mana, miracles relied in the faith and belief in gods. As the gods have long since ceased to exist, at least in the eyes of the public, so have miracles likewise ended.