I think MC got some affinity to magic, but was chosen randomly by Al'Azar. I mean that MC wasn't destined to be his "knight", but had needed affinity.
The thing is, MC is clearly special and exceptional. He's the only male human user of lights, ever,
except for Sally's former master(?), but it's also implied MC may be the same person as him/reincarnation. Additionally, MC is exceptional among light users generally, after only a week he can match skilled demons like Keera. He can also completely resist (mental?) lights that are cast against him, which afaik no other light user or demon can do
There's also the assumption that MC has a blood relation to Karen, a coven leader, so MC has a magic bloodline or something
There has to be
some reason this is the case, rather that just by random chance or arbitrary choice.
A character who I can't remember (Titus?) comments that the barrier between Earth and demon world is weakening (due to Al'Azar's awakening/ritual) as a partial explanation but doesn't explain why MC and only MC is the only male to get super magic powers. The Succubus could very well be using the MC because she knows of his special status (other than being male) which may be the only way to stop Al'Azar.