Well the Merchants from the Hub told us a bunch of caravans disappeared on their way up north. I think there's an army in the mountains, but the Elders, well, they don't want to act until they're sure.
If the Vault Dweller want help at the Cathedral, he must then go to Cathedral, enter the hall and go upstairs to the 4th floor. Deal with Morpheus and hack his computer.
After this had been done, return to the Brotherhood. Report to Maxson and convince the High Elders about the danger from the base.
If the Vault Dweller was at the Cathedral and mention the control from it, they will send assistance at his discretion; either to the base or to the Cathedral. Otherwise the Vault Dweller can only take help to Mariposa.