Cult of the Lamb

Both this achievement and Weapons of Plenty (30G) are unlocked via the Sermon unlocks that you get from building up enough points from giving Sermons in your Temple. Along the left side of this upgrade menu are the unlocks for the various types of weapons (Fervor, Godly, etc.), and along the right side are unlocks that add three more Curses to your game for each unlock. So to unlock Weapons of Plenty (30G) you just need to unlock all types of weapons up the left side of the screen, and for this achievement you need to unlock all of the Curses along the right side of the screen.

Getting the points for Sermon upgrades come from two places:
 

1. Sermons themselves, obviously. The more Followers you have, the more points you get per Sermon.
2. Sacrificing Followers, less obviously. Specifically, using the Sacrifice Ritual. The higher the loyalty of a Follower, the more points you get per sacrifice.

In case you were wondering why you'd sacrifice a Follower, it's to get massive points for Sermon upgrades. To put it in perspective, you'll likely be getting somewhere in the 15-25 point range for Sermons, but Sacrificing a Follower can easily net you 60-100 points. On top of that, there's an achievement (Weigher of Souls (50G)) that requires you to sacrifice 20 Followers anyway, so you should absolutely take advantage of that early in your playthrough to both make progress toward that achievement but also to get better and more weapons and Curses available to you early on.