While driving around with Trevor you will, from time to time, get a Strangers and Freaks mission come up where someone needs a lift somewhere, then top left you will get a pop-up explaining that you can take these passengers to the Altruist Cult for a monetary reward.
The cult location is in the hills on the West side of the map, the icon should show up during these missions.
Simply drop the people off to the cultists and the achievement will pop