Judas

Judas is an unlockable character in The Binding of Isaac. He is unlocked by completing The Womb or Utero for the first time.

Judas starts with The Book Of Belial, higher than average damage and only one heart container. This makes him the "glass cannon" type character of the game, meaning he is more powerful but also more fragile than other characters early on.

Strategy

General

More so than for any of the other characters, defensive items are crucial for a successful Judas playthrough. Bomb tinted rocks whenever you see them to collect Soul Hearts, and generally avoid making deals with the devil unless you've got enough health to survive comfortably after a deal.
Ironically however, Judas has one of the best chances of all characters in the game of spawning a Devil Room (91% if no damage was taken on that floor while holding The Book of Belial). You can then turn down deals from the devil to get a chance for Angel Room items in the Wrath of the Lamb expansion, which are of no consequence to your health.
With The Book of Belial, he can take down any of the bosses in The Basement or The Caves within a minute or two. Don't be afraid to keep the book until late floors.
Items and pill effects that increase his rate of fire make him even more deadly.
Judas's three starting coins don't have much late-game impact and aren't terribly useful (certainly less so than a bomb or a key), but they can give you a nice starting boost if a Shop or gambling-heavy strategy becomes viable. At worst, they can always be chucked into a slot machine or given to a beggar for the chance of a pickup or an item.

Items

As mentioned previously, avoid devil room deals early on. You need all the health you can get as Judas.

Best Items:

The Book of Belial
Any health upgrades (Dinner, Raw Liver, MEAT!, etc.)
Book of Revelations
The Nail
Spoon Bender
The Wafer
The Battery/9 Volt
Cancer (Trinket)
Mom's Pearl (Trinket)

Good Items:

A Pony, White Pony, Transcendence, or Lord of the Pit
Cupid's Arrow (for fast damage)
Steam Sale
Deck of Cards
The Hourglass
Ouija Board
Halo Of Flies (For early defensive means)
Book of Shadows
The Gamekid
My Little Unicorn
IV Bag (only if you have heart pickups to spare, or invincibility granting items such as The Gamekid , My Little Unicorn, or Book of Shadows )
Dead Cat (if you only have one heart container, as it will essentially function as extra lives early in a playthrough)

Bad Items:

Infestation (Initially)
Holy Water (Initially)
Lemon Mishap (Initially)
Razor Blade
Blood Rights
Kamikaze!

Related Achievements

Judas' Fez - Defeat Mom's Heart once.
The Robo-Baby - Kill Mom's Heart with Judas.
The Monster Manual - Complete Sheol with Judas.
The Curved Horn - Complete Cathedral with Judas.
Judas' Tongue - Complete The Chest with Judas.
Eternal Judas - Complete The Chest with Judas on Hard Mode.

Gallery

Trivia

The three pennies he starts out with are likely a reference to the 30 pieces of silver given to Judas for his betrayal of Jesus in Bible scripture.
The fact that Judas starts out with low HP and a Devil item suggests that Judas made a deal with the Devil, which costs hearts and gives you demon items.
The Unholy Edition art booklet calls Judas the "mage" with reduced HP. It also refers to his starting item as "Necro"; Judas was originally supposed to start out with the Necronomicon instead of the Book of Belial.
Judas' fez hat is a reference to Phil Fish, (creator of FEZ) who voted against Edmund McMillen and his partners during the Independent Games Festival (IGF), therefore becoming "Judas" (a traitor).
- Edmund McMillen has stated that the fez was because he needed an iconic hat for Judas, and he went with the idea of Phil Fish as a "Judas" because it seemed funny.
When the player pauses the game right as they enter the Basement XL, Judas will appear as if he has The Pony. This goes away as soon as the pause screen goes away.

Characters
Isaac • Magdalene • Cain • Judas • Eve • ??? • Samson