Right, But lets take 2 Black mages of the same Zodiac sign. level and MA on the same battlfield.

Black mage 1 has 70 Faith and Black mage 2 has 40 Faith. BM 1 will do more magic damage because his faith is higher. If you take a mediator and raise BM 2's faith to 80 BM 2's magic will be more potent.