From Final Fantasy Wiki

Faith is a statistic from Final Fantasy Tactics. It is a measure of a character's belief in God, and affects their Magic Attack and Magic Defense scores. However, it also makes magical attacks more effective against that unit.

Female units tend to have higher Faith scores than Male units, and units with high Faith scores are suitable to become mages. However, any character whose Faith stat becomes too high will leave Ramza Beoulve's party to go seek God on their own. Ramza himself is an exception to this, and can safely have his Faith raised as high as the player wishes.