To start the Altruist Cult mission in Grand Theft Auto V, follow these detailed steps:

Step 1: Ensure Trevor is Affiliated with the Altruists
– Complete Nervous Ron: Before you can participate in the Altruist Cult missions, you need to complete the mission “Nervous Ron” where Trevor takes over the Azteca’s Airfield and does his first gun-running run.

Step 2: Identify Hitchhiker Encounters
– Random Encounters: After completing Nervous Ron, you will start encountering hitchhikers who are part of the Altruist Cult. These encounters can happen randomly while driving around the map.

Step 3: Pick Up Hitchhikers
– Select Hitchhikers: When you encounter a hitchhiker, you can choose to pick them up. The hitchhiker will have a specific destination, which is the Altruist Cult camp. You can pick up any hitchhiker except for those associated with specific missions.

Step 4: Deliver Hitchhikers to the Altruist Camp
– Drive to the Camp: Once you’ve picked up a hitchhiker, drive them to the Altruist Cult camp. A temporary marker will appear on your map to guide you to the camp.
– Drop Off Hitchhikers: As you drop off each hitchhiker at the camp, you’ll see a cutscene where the Altruist Cult members take the hitchhiker into the camp. This process can be repeated multiple times, but you need to deliver four hitchhikers non-consecutively to trigger the special event.

Step 5: Trigger the Special Event
– Fourth Delivery: On your fourth delivery, the Altruists will “kidnap” Trevor and take him into the camp for a special cleansing. This is when the Altruist Cult Shootout event begins.

Step 6: Complete the Altruist Cult Shootout Event
– Activate Rage Ability: After the cutscene, quickly activate Trevor’s Rage ability to gain an advantage in combat.
– Clear the Area: Stay in cover behind the large rock and wait for the other Altruists to come running towards you. Pick them off one by one until all that’s left are the gunners above.
– Eliminate Gunners: Equip the Sniper Rifle and pick off the two Altruists perched on the water tower to the south and the lone Altruist on the rooftop on the west side.
– Final Altruist: Eliminate the last Altruist by the watch tower at the entrance to complete the event.

Step 7: Collect Briefcases (Optional)
– Collect Cash: After clearing the area, you can collect four briefcases each containing $25,000. These briefcases only spawn once during this event, so make sure to grab them before exiting the camp.

By following these steps, you will be able to start and complete the Altruist Cult mission in Grand Theft Auto V.