Complete the mission "Walking With Spirits". Mission can only be done after "The Local Flavor" mission, and will be received upon completing said mission. When you arrive at the base of the Tribals (the Cathedral), they will ask you to go into the Sacred Bog (far west) and get seeds from the big Punga fruit in the Grotto. This is your Rite of Passage to gain entrance to the Tribals. Upon arriving in the bog, you will have to follow the river and rocks towards the giant Punga fruit, all whilst fighting various Mirelurks. After activating the Punga, travel back through the bog, amongst a bunch of weird goings-on. You reach the end when you meet a man standing by the bomb that's in the middle of Megaton. You then regain consciousness and have to trek your way back to the Cathedral.