This achievement is exactly what it says it is, have 666 gold coins at once.
- A quick way, is that if you have a bunch of resources that you no longer want, you can sell them all through the Offering Chest.
- If you don't want to sell all your stuff, once you have fully unlocked everything from the Shrine, the additional devotion collected turns into gold coins. So if you want to wait until you have everything unlocked, this is a quick and easy way to make a lot of gold coins especially, if you have a lot of followers and have them all pray.
- Additionally if you have unlocked Midas's Cave, there is a well there that you can deposit money into and come back a few days later to and get more gold coins out than you put in. However, this is not as quick as the above methods.
- I'm not very familiar with this method but there are rituals that you can perform and doctrines that you can declare to get more gold coins from your followers. I'm not sure how quickly these get you gold coins, and what affects these will have on your followers.
- There are other ways to get a gold coins such as Knucklebones, or naturally gaining coins from crusades, which can also take a while to gain gold coins.