"Basement Boy" - Finish the two Basement/Cellar levels without taking any damage. (You just need to kill the bosses, you don't have to clear all of the rooms).
"Spelunker Boy" - Finish the two Caves/Catacombs levels without taking any damage. (You just need to kill the bosses, you don't have to clear all of the rooms).
"Dark Boy" - Finish the two Depths/Necropolis levels without taking any damage. (You just need to kill the bosses, you don't have to clear all of the rooms).
"Mama's Boy" - Finish the Womb/Utero levels without taking any damage. (You just need to kill the bosses, you don't have to clear all of the rooms).
Halloween Update
(Some of these may be wrong)
"The Razor" - Finish The Womb with Eve. (Activated item)
"Mom's Knife" - Kill Satan with Isaac. (Collectible Item)
"Eve" - Make 2 Separate Devil Room Deals in one playthrough. (Playable Character)
"A Curse" - Make 2 Separate Devil Room Deals in one playthrough. (Collectible Item)
"A Dead Bird" - Make 2 Separate Devil Room Deals in one playthrough. (Collectible Item)
"A Bag of Bombs" - Kill Satan with Cain. (Collectible Item)
"Guardian Angel" - Kill Satan with Maggy. (Collectible Item)
"A Demon Baby!" - Kill Satan with Eve. (Collectible Item)
"A Forget Me Now" - Kill Satan with ???. (Activated/Consumable Item) [Takes up your item slot and dissapears when used]
"Golden God!" - Unlock all secrets and complete the Collection.This achievement can be gained by collecting all but one of the items if you have defeated Krampus and found the lump of coal.
Title screen with Golden God achievement unlocked.
Wrath of the Lamb In-Game Secrets List
Characters
"Samson" - Complete Depths 2, but pass on 2 Treasure Rooms. (Playable Character)
Game Completion Unlocks
"The D20!" - Complete the Cathedral as Isaac. (Activated Item)
"The Crucifix" - Complete the Cathedral as Maggy. (Collectible Item)
"Abel" - Complete the Cathedral as Cain. (Collectible Item)
"Curved Horn" - Complete the Cathedral as Judas. (Trinket)
"Sacrificial Knife" - Complete the Cathedral as Eve. (Collectible Item)
"Rainbow Baby <3"Complete the Cathedral as ???. (Collectible Item)
"Bloody Lust" - Complete Womb 2 as Samson. (Collectible Item)
"Blood Rights" - Complete Sheol as Samson. (Activated Item)
"Bloody Penny" - Complete the Cathedral as Samson. (Trinket)
"The Polaroid" - Complete the Cathedral six times. (Trinket)
"You Are Eternal" - Complete The Chest as Isaac in Hard Mode.
"You Are Eternal" - Complete The Chest as Maggy in Hard Mode.
"You Are Eternal" - Complete The Chest as Cain in Hard Mode.
"You Are Eternal" - Complete The Chest as Judas in Hard Mode.
"You Are Eternal" - Complete The Chest as Eve in Hard Mode.
"You Are Eternal" - Complete The Chest as ??? in Hard Mode.
"You Are Eternal" - Complete The Chest as Samson in Hard Mode.
Task Based
"Eternal Mom" - Defeat Mom in Hard Mode.
"Eternal Heart" - Defeat Mom’s Heart/It Lives in Hard Mode.
"Eternal Life!!!" - Gain 7 health upgrades from Eternal Hearts in Hard Mode.
"Eternal Satan" - Defeat Satan in Hard Mode.
"Eternal Isaac" - Defeat Isaac in Hard Mode.
"Eternal Blue..." - Defeat ??? in Hard Mode.
"Eternal Personalities" - Complete The Chest with every character in Hard Mode.
"!!Eternal God!!" - Unlock all other secrets.
"Eternal God!" - Complete the game. (New backdrop)
Bugs
Secrets might be missing even though they have already been earned, due to various Anti-Virus programs, CCleaner (Or similar cleaner programs) or Steam's Cloud Sync.
For a detailed description on how to fix the missing secrets, check the video below.