Become an Initiate.

Become an initiate is a side quest in Fallout.

Synopsis
The Vault Dweller expressed interest in joining the Brotherhood. Great! Initially jubilant and welcoming, Cabbot's much more reserved after he returns from his meeting with the Elders. In order to be permitted to join, they must complete a quest: Recover a holodisk from the 'ancient order' far to the south-east. Then and only then will they be accepted as a member.

Walkthrough

 * Journey to Lost Hills and talk to Cabbot about joining. Accept the quest once he returns from the Elders.
 * Characters with less than 4 Intelligence cannot talk to Cabbot about joining and cannot complete the quest. Darrel will instead joke about their intelligence, then mark the location on their map and even give one Rad-X to help with looting the crater.
 * Purchase a rope and 2 doses of Rad-X before leaving for the Glow.
 * Ropes are sold by Mitch, Seth, and can be found for free in Shady Sands.
 * Rad-X is also sold by Mitch, as well as Vance. Six can be taken from an ice chest of the ground floor of the Followers' library.
 * Head to the Glow. Enter inventory upon arrival and take 2 Rad-X to gain 100% radiation resistance.
 * Use the rope on the protruding beam above the crater. Use the rope to descend.
 * Loot the dead soldier in power armor for the ancient Brotherhood disk, then return to Cabbot.

Rewards

 * 2000 XP
 * Becoming an initiate of the Brotherhood of Steel and access to Lost Hills