Cult of the Lamb

This achievement relates to the Divine Interventions you unlock from getting enough Devotion from your Followers. Note that you do NOT need to unlock every single building and whatnot with Divine Interventions. All that is required for this achievement is to unlock enough buildings and upgrades such that you can access the final tier on top, and then purchase that final Cult upgrade at the top.

You'll quickly find that, every single time you come back from a Crusade run, your Shrine is full of Devotion and needs to be emptied so your Followers can continue filling it. So while it will take many in-game days (likely 30-35 or so, just as a very rough estimate) to get enough Devotion from your Followers to unlock this achievement, there is plenty to do for progression and other achievements. To speed it up, simply minimize the amount of Followers that have other jobs. For example, there's no real reason (for achievements or otherwise) to build the majority of the structures available and set your Followers to work. I had one person tend a farm, one or two to chop wood/mine rocks, and one to consecrate resources (via the refinery unlocked later), and that was it. Everyone else was worshipping all the time.

Since your Shrine will continually fill up, obviously upgrade your Cult and therefore the Shrine as soon as possible. The Tabernacle is another place for workers to deposit Devotion, but it's not really worth it since it holds such a small amount compared to your upgraded Shrine. Just make sure to pop up once per day and empty the Shrine.

One other (very fast) way to get Devotion is via Fervor Weapons. These types of weapons, unlocked via the Sermon upgrades, collect all that red Fervor that enemies and chests drop all over the place and convert it into Devotion. Weapons are completely random, so you can't rely on this entirely, but you'll assuredly get a Fervor weapon a few times in your Crusades. To put it in perspective, a full Crusade run with a Fervor weapon was netting me a solid three Divine Interventions to redeem when I got back to camp, as opposed to waiting two in-game days to get one Divine Intervention. So take full advantage each time you get a Fervor weapon to explore every room of each stage, pick more combat stages, and ensure you get all the Fervor from chests and the boss, and you'll see your Devotion meter go up like crazy on those runs.

On a related note, once you unlock this achievement, there are no other achievements that require you to unlock more or all of the buildings and upgrades. Devotion is therefore not really needed anymore, so don't feel any further need to continually empty it and make sure you're getting more.