Details for the Diplomatic-Infiltration Approach

Enter the Cathedral in your Power Armor.

Speak with the man in leather armor in the main prayer area and barter for his red Cathedral key.

Use the red Cathedral key on the locked door next to the lady in the leather jacket.

Climb the stairs to the fourth floor.

Equip a combat shotgun or .223 pistol in an inactive weapon slot.

Open the door to Morpheus' room, enter combat, and kill him.

Dispose of his three Nightkin guards (two with flamethrowers, one with a minigun).

Examine Morpheus' corpse and take his black Cathedral key.

Return to the first floor, put on your robes, and equip the black Cathedral key.

Use the black Cathedral key on the locked door with a bookcase to enter the stairwell to the basement.

Descend into the Cathedral basement and locate the secret door to the Cathedral vault.

To open the vault: either wait for Jeremiah (Master's messenger) to open the door, or examine the bookcase to the left of the secret entrance, use your traps skill (38% with 2 tries and 5 luck worked for one player), and then examine it again to open the door.

Enter the cavern entrance to the vault, where you will encounter floaters and centaurs that will ignore you due to your robes.

Proceed to the north corner entrance of the vault, guarded by two Super Mutants. State you are on important business and give them a blessing to pass, earning 750 xp.

Use Science on the computer next to the elevator to find the military base location, earning 1250 xp.

Enter the elevator and go to Level 2.

On Level 2, go to the room with a lone Super Mutant guard and scientists, then go south to find trapped prisoners.

Use the terminal on the wall to free the prisoners, earning 2000 xp. (Note: The prisoners will be blown away after reaching the terminal).

Go to the northwest of Level 2, enter the room left of the prayer room, and speak with the man in the green shirt to ask for a nullifier, receiving 1000 xp. Equip the nullifier in your active item slot.

Return to the elevator for Level 3.

On Level 3, head south, turn right, and proceed towards the control area with goo-covered computers.

Open the door, go through the passageway, and speak with the Master.

Engage in the dialogue with the Master. Choose to help him if he proves the Unity is the best course for humanity. When he asks you to prove it, state you can't know unless he proves it. When he states the Unity will bring about the Master race, point out it will be mutants. When he says all who resist will be changed, state you know mutants are sterile. When he claims FEV-2 doesn't destroy reproductive organs, ask if he's spoken to his mutants. When he asks what you say to his scientists' assurance, ask about a female mutant. When he realizes his race is doomed, state it's not an option for him. Choose 'done' for his last speech and leave the Cathedral immediately.

Run to the elevator, descend to the first level, and exit the Vault cavern through the secret door and basement.