On this page of the game guide to Borderlands 2 you will find a detailed description of the side mission titled Cult Following: Eternal Flame. You can start this quest in the Sanctuary location after completing the main mission named A Dam Fine Rescue.
2) Clayton/Fire Effigy
3) Bandit camp
5) Fire Effigy 2
6) Fire Effigy 3
Inside Roland's hideout, climb to the first floor and speak with Lilith on the balcony . You will have to investigate the case of a cult, which worships her as a goddess. Head to Frostburn Canyon by using Fast Travel. Go north and speak with the cult leader, Incinerator Clayton . Your first task will be killing five Bandits in the camp  by setting them on fire with a weapon with Fire Elemental Effect. Some of the enemies will be resistant to fire. In such case, firstly reduce their health with another weapon and finish them off with one with a Chance to Ignite. If you manage to do it, you will be able to pick up the Bandit ashes shown in the screenshot. Collect five of them and return to Clayton .
Now you will have to kill Scorch . Head to the tunnel and follow the eastern branch at the first fork, in accordance with the dots on the map. You will reach an open space where you will face your target. The enemy is of course resistant to fire and can also create a force wave which will set you of fire (screenshot). Whenever you see a ring of fire around him, run as far away as you can. With Scorch killed, return to Clayton .
After the conversation, open the toiler to the right of Clayton and "pick up" Matchstick. Now return to Fast Travel and move to Southern Shelf.
You will have to reach a ship in the north of the map, where you fought with Flynt. Once on board, head to the right side, where you will first have to hand hang Matchstick and afterwards pull the lever shown in the screenshot. Now return to Clayton . For a quick shortcut to Fast Travel, you can jump down from the right side of the ship deck.
The next stage of this mission requires you to light up three Fire Effigies shown in the above screenshot. The first one can be found in the camp, nearby Clayton . The other two are hidden in Bandit camps  and . You however don't have to approach them very close if you have a weapon with a good magnification. With all targets on fire, Clayton will contact you through radio and give another task .
Head through the tunnel along the dots.
Once you reach the destination point, kill all the enemies around and use the lever shown in the screenshot. Afterwards kill Clayton and all the Cultists to end the mission. Turn it in to Lilith .