Ive beaten fallout 1 before a few years ago and i don't remember having this problem, i know he's under the cathedral but ive done everything else and talked to everyone i can, but i haven't been able to actually find anyone ingame who gives you the location of the cathedral and the master

what am i missing?

Is long time since I played this game for last time, but:
You talked with the priestess of the children in The Hub?
And with the leader of the leader followers of the apocalypse in L.A. Boneyard? She directs you to de cathedral, to talk with a spy that is collecting information about the children.

Every single person in the Boneyard brings up "Hey, have you seen the big Cathedral nearby?"