Crusader Kings III

The long way to get this is to become an Emperor and form a religion with the Natural Primitivism Tenet. Picking up the Perk Prophet will make the creation of the religion cheaper. You can also convert to the Adamitism religion which gets easier as the game goes on but at the beginning is quite expensive in Piety - the Apostate Perk will make this cheaper.

For a quick and easy way to acheive this start as the Holy Roman Emperor in 1066 or the Byzantine Emperor in 867 (Win the war that you start with down at the bottom of Italy. Destroy the army as it lands and Enforce Demands or White Peace it doesn't really matter)

As soon as the war is over if you are playing the Byzantine Emperor use the Decisions to go on a Pilgrimage (try one of the longer ones) and what you are hoping to see popup is a Naked Adamite. This will give you the choice of converting to the Adamite religion for little cost and as soon as this happens you will get the acheivement. This can take a few attempts as it is not guaranteed but the whole process can be done in 5 minutes.

The other much less likely way is to go insane and get a choice to be naked. You can increase the chance of insanity by getting stressed but definitely not a reliable way to do it.

You can complete this achievement in 10 minutes using the Ruler Designer. Steps:

1. In the 867 start date choose Basileus Basileios (Emperor of the Byzantine Empire), and choose to create your own ruler.

2. Make your ruler 120 years old. For traits give him Robust, Pure-Blooded, Strong, and Athletic. Make his education trait Mastermind Philosopher. Make his personality Cynical. Orthodox Religion and no family is fine. Get his Learning as close to 30 as you can and make everything else as low as possible.

3. Make sure you're in Ironman and start the game. Once you're in immediately put your "stockpiled" 100 years of education into Theologian and Whole of Body (This is why you start so old. The game gives you what it thinks you would have earned from age 16, so 104 years is a lot). I chose Medicine focus, but Theology is probably fine.

4. For me with Cynical and 27-30 Learning, I could convert to Adamitism for 250 Piety (down from 12650). You start with around 150 piety, so just wait until you accrue 100 more (You will likely get Lifestyle events as well, take the piety options). I was able to get the achievement in 867, about 10 months total.

Easy way:

1. Pick an existing emperor or use a save you might have.
2. Press [C] and search for Adamite, as they don't wear clothes.
3. If opposite gender, invite to court (seducing is the easiest option).
4. Right Click on the Adamite [Adopt Faith].

Easy way 2:

1. Locate Adamite through [C].
2. Send one of your children as ward.
3. Chose [convert to .... faith].
4. Adopt your child's faith once it returns.

Easy way 3:

1. Press [R].
2. Find Adamitism on the map.
3. Click on the faith and chose [convert to faith].