Convince the Elders to send help

 is a side quest in Fallout.

Quick walkthrough

 * 1) Go to Mariposa Military Base and discover the mutants inside.
 * 2) (Optional) Go to the Cathedral, make your way into Morpheus' room and get Lieutenant's letter from the computer.
 * 3) Return to Lost Hills and inform Elder Maxson of the danger.
 * 4) Convince the Elder to send Paladins.

Detailed walkthrough
Go to Mariposa Military Base and exit. If you want help to Mariposa, skip to Brotherhood.

If you want help at the Cathedral, you 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 you were at the Cathedral and mention the control from it, they will send assistance at your discretion; either to the base or to the Cathedral. Otherwise you can only take help to Mariposa.

Rewards

 * Either 3 assisting Paladins at Mariposa Military Base,
 * Or 2 assisting Paladins at Cathedral,
 * 1500 XP.