the church sees the job system as something sacred
-> the blacksmith job can eventually (presumably) craft weapons just as good as the sacred weapons
-> therefore, the blacksmith job allow people to choose a different path from the assigned job system
-> defying the job system is the same as defying god/church
-> blacksmith job is sacrilegious
-> the church bad mouths the job publicly to put down any holders of this profession
Either that, or MC's blacksmith job is actually unique and far stronger than the regular blacksmiths