There isn't, but it's not especially deep, either.
ATK is attack and affects physical damage, and is reduced by DEF, which is defense.
MAT and MDF are magic attack and magic defense, and have the same relationship.
AGI is agility and affects the order characters take actions in combat.
LUK is luck and has minor effects on things like evade rate, hit rate, and crit rate.
The only Theurgy that's kinda weird is Treatment, which takes into account the MDF of the user and the target instead of only the user's MAT. Everything else works pretty much how you'd expect it to, though there's more specific info on things like how buffs work in the README.