Purist

Purist is challenge #20. This challenge is unique because Isaac starts with no items. The goal is to defeat Mom's Heart or It Lives. This challenge is unlocked after defeating Mom for the first time.

Difficulty

Because there are no Treasure Rooms, it is harder to find items. While this is standard for most challenges, Isaac also doesn't start with any items.

Strategy

Other special rooms become much more important, with Devil Rooms and Angel Rooms becoming even more valuable than on an average run.
Cards such as Credit Card and items such as Diplopia, Coupon and Mr. ME! can be used for the Shop and Devil Room items, with Diplopia cloning the items, and Credit Card/Coupon allowing the item(s) to be taken for free. Neither option stops Angel Rooms from appearing.
Bombs are much more valuable when hunting for Secret Rooms as these can provide free items.
Curse Rooms guarantee damage and can lower the chances of Devil Rooms. However, if Isaac has a full Soul Heart, Black Heart, or Bone Heart, the reward can outweigh the damage and still allow Isaac to access a Devil Room. Alternatively, wait to enter the Curse Room until after visiting the Devil Room.
The Bible can be used to instantly win the boss fights against Mom, which can be used to preserve key health, and Mom's Heart/ It Lives, which will complete the challenge.