While activated, switching items will end the bonus damage effect.
If the Schoolbag and a second activated item are held, the book must be equipped in the primary slot to receive the Devil Room / Angel Room bonus.
Only guarantees Devil/Angel Rooms on floors that they can normally appear on, and, unlike Goat Head, picking up The Book of Belial after killing the boss will not open the Devil/Angel Room door.
The damage bonus does not stack with multiple uses in the same room or with XV - The Devil.
Additional uses in the same room or combined with XV - The Devil give +1 damage per use.
Synergies
Blood of the Martyr: Causes The Book of Belial to apply a 1.5× damage multiplier on top of its normal effect. This multiplier cannot stack with multipliers from Cricket's Head or Magic Mushroom.
Book of Virtues: Spawns black and red wisps in the middle ring. Wisps deal double damage compared to normal wisps.
Car Battery: Gives +3 damage for the current room.
While holding Birthright with Judas or Black Judas, The Book of Belial is able to be held alongside another active item, similarly to Book of Virtues. The book provides +0.66 damage for each charge of the held item. Items that do not require charges to use do not grant a damage bonus unless there is an exception listed below.
Certain active items also gain additional effects, which are noted below; any items not listed have their standard effects aside from the temporary damage up Birthright gives them.
Book of Virtues: Can be stacked along with The Book of Belial, allowing you to carry 2 passive books and an activated item on top of them.
Breath of Life: Judas leaves a damaging flame trail while the invincibility is active instead of creating beams of light. Blocking damage just as the invincibility begins activates The Necronomicon effect and grants Judas a temporary damage bonus.
Clicker: Replaces the most recently acquired item with Pentagram instead of removing it. Birthright's effects will also change to that of the character transformed into.
Damocles: The sword also increases Judas' damage by x1.4 then a flat +1 while it's active.
Dark Arts: Grants temporary damage ups for every enemy/projectile hit by Dark Arts, similarly to Tainted Judas. This stacks with Black Judas' inherent version of this, doubling the amount of damage gained.
Decap Attack: Grants Judas +2 damage and ×3 tears fired per second while his head is detached from his body.
Forget Me Now: Grants +2 damage up for the entirety of the new floor.
Genesis: Grants a free Pentagram that will not be removed by Genesis.
Glass Cannon: Grants Judas +2.67 damage up while Glass Cannon is broken.
Golden Razor: Also invokes the effect of Magic Fingers when used, which occurs after the damage increase and deals an additional +5 damage; non-boss enemies killed by this effect also have a chance to drop 1-2 pennies.
Guppy's Paw: Grants a permanent 0.5 damage up for every converted heart.
How to Jump: Enemies Judas jumps through take 2 + 0.2 × floor contact damage and are set on fire. The burn deals damage equal to Judas' damage, and ticks up to 5 times.
Isaac's Tears: Fires a ring of 10 tears instead of 8, the tears are piercing and spectral, and the tears deal 1.4 × Judas' damage, plus a flat 5.
IV Bag: Gives a temporary +0.66 damage up with each use.
The Jar: Instead of dropping hearts on the floor, using The Jar consumes up to one full heart in the jar at a time, giving a temporary +0.66 damage up for each half heart consumed and spawning a puddle of red creep that deals 2 damage per tick (20 damage per second).
Magic Skin: Grants a permanent +0.5 damage up on use in addition to the temporary +4 damage up given to it by Birthright. This bonus is not lost if the broken hearts are removed.
Notched Axe: The axe receives a purplish glow and receives the following upgrades:
Increased durability: Each use of the axe has a very high chance not to use a charge.
Faster usage: The axe can be swung up to four times per second, regardless of Judas' tears stat.
Higher knockback: Enemies are knocked back when hit, receiving 8 + 0.2 × floor damage if they collide with a wall, obstacle, or other enemies.
Fire effect: The axe will set enemies on fire. The burn deals damage equal to Judas' damage, and ticks up to 5 times.
Increased luck: destroying obstacles with the axe has an increased chance to spawn pickups.
Destroying Poops will spawn pickups much more frequently. When combined with Petrified Poop, the chance becomes 100% and will often result in multiple pickups from a single poop.
Destroying Props (such as Urns, Mushrooms, Skulls, and Polyps) gives the prop an extra chance to drop its rewards. This can result in a single prop giving multiple drops.
Destroying Fool's Gold rocks will drop significantly more coins.
Sacrificial Altar: Grants +10.8 temporary damage up for every sacrificed familiar. The damage bonus scales down to 0 over time, similarly to Red Stew. Sacrificing blue flies or blue spiders has no special effect.
Sharp Key: Invokes the effect of Razor Blade without dealing damage to Judas along with the original effect.
Stitches: Creates a shadowy line similar to Dark Arts between Judas and the familiar. Enemies caught in the line take damage equal to 2 × Judas' damage, plus a flat 3.
Urn of Souls: The stream of fire is replaced with a controllable Brimstone beam that destroys projectiles and deals more damage the longer it's active. The minimum damage is 3.37 + 0.361 × floor, and the maximum damage is 4.2 + 0.45 × floor, which it scales up to over the course of 3 seconds of continuous fire. Souls dropped from enemies are colored red, and the Urn displays a red graphic on the HUD when active.
Vade Retro: Instead of exploding on the spot, souls home in on nearby enemies and explode on contact, similarly to Purgatory. The souls deal 2 × Judas's damage up to 3 times and destroy projectiles.
Wavy Cap: Gives a damage bonus which lasts until the next floor equal to 0.2 × the maximum number of times Wavy Cap's effects have been stacked.
The previously unused item ID 59 is now occupied by the active/passive version of The Book of Belial, which is obtained when Judas or Black Judas has Birthright.
Despite this, this version of the item is not obtainable by other characters. If you use Spindown Dice on an item pedestal of item ID 60, The Ladder, it'll skip to Book of Shadows.
The effect of Notched Axe with Judas' andBlack Judas' Birthright is a reference to Minecraft's enchanting mechanic.
All of its upgrades are based on existing Minecraft enchantments — Unbreaking, Fire Aspect, Knockback, Efficiency, and Fortune.
However, Fire Aspect and Knockback normally cannot be applied to pickaxes in Minecraft.
Satanic Bible.
Guarantees the 
Devil Room or 
Angel Room to spawn after killing a boss while holding it.
Increases the chance to open the 
This item belongs to the 
Bookworm set. Collecting three items from this set will transform Isaac into Bookworm.
Judas starts with this item by default.
While activated, switching items will end the bonus damage effect.
Schoolbag and a second activated item are held, the book must be equipped in the primary slot to receive the 
Goat Head, picking up The Book of Belial after killing the boss will not open the Devil/Angel Room door.
XV - The Devil.
Additional uses in the same room or combined with 
Blood of the Martyr: Causes The Book of Belial to apply a 1.5× damage multiplier on top of its normal effect. This multiplier cannot stack with multipliers from 
Cricket's Head or 
Magic Mushroom.
Book of Virtues: Spawns black and red wisps in the middle ring. Wisps deal double damage compared to normal wisps.
Car Battery: Gives +3 damage for the current room.
Birthright with 
Black Judas, The Book of Belial is able to be held alongside another active item, similarly to 
Alabaster Box: Changes the payout to 2 
Black Hearts and 1 item from the Devil Room item pool. If a Devil Deal has been taken, the payout is boosted to 3 Black Hearts and 2 Devil items.
Bag of Crafting: Collecting 
Red Hearts, 
Half Red Hearts, or 
Double Hearts with this item has a chance to transform them into 
Blood Rights: Invokes the effect of 
Razor Blade along with its original effect.
Breath of Life: Judas leaves a damaging flame trail while the invincibility is active instead of creating beams of light. Blocking damage just as the invincibility begins activates 
The Necronomicon effect and grants Judas a temporary damage bonus.
Clicker: Replaces the most recently acquired item with 
Pentagram instead of removing it. Birthright's effects will also change to that of the character transformed into.
Damocles: The sword also increases Judas' damage by x1.4 then a flat +1 while it's active.
Dark Arts: Grants temporary damage ups for every enemy/projectile hit by Dark Arts, similarly to 
Tainted Judas. This stacks with 
Decap Attack: Grants Judas +2 damage and ×3 tears fired per second while his head is detached from his body.
Forget Me Now: Grants +2 damage up for the entirety of the new floor.
Genesis: Grants a free 
Glass Cannon: Grants Judas +2.67 damage up while Glass Cannon is broken.
Golden Razor: Also invokes the effect of 
Magic Fingers when used, which occurs after the damage increase and deals an additional +5 damage; non-boss enemies killed by this effect also have a chance to drop 1-2 pennies.
Guppy's Paw: Grants a permanent 0.5 damage up for every converted heart.
How to Jump: Enemies Judas jumps through take 2 + 0.2 × floor contact damage and are set on fire. The burn deals damage equal to Judas' damage, and ticks up to 5 times.
Isaac's Tears: Fires a ring of 10 tears instead of 8, the tears are piercing and spectral, and the tears deal 1.4 × Judas' damage, plus a flat 5.
IV Bag: Gives a temporary +0.66 damage up with each use.
The Jar: Instead of dropping hearts on the floor, using The Jar consumes up to one full heart in the jar at a time, giving a temporary +0.66 damage up for each half heart consumed and spawning a puddle of red creep that deals 2 damage per tick (20 damage per second).
Jar of Wisps: Will only spawn 
Kamikaze!: Invokes the effect of 
Magic Skin: Grants a permanent +0.5 damage up on use in addition to the temporary +4 damage up given to it by Birthright. This bonus is not lost if the broken hearts are removed.
Notched Axe: The axe receives a purplish glow and receives the following upgrades:
Poops will spawn pickups much more frequently. When combined with 
Petrified Poop, the chance becomes 100% and will often result in multiple pickups from a single poop.
Pandora's Box: In addition to the item's expected reward, spawns one of the following demonic familiars: 
Blood Puppy, 
Dark Bum, 
Demon Baby, 
Incubus, 
Lil Abaddon, 
Lil Brimstone, 
Succubus, or 
Twisted Pair.
Plan C: Respawns Judas as 
Potato Peeler: Each use grants a permanent +0.5 normal damage up in addition to a +0.2 flat damage up.
Sacrificial Altar: Grants +10.8 temporary damage up for every sacrificed familiar. The damage bonus scales down to 0 over time, similarly to 
Red Stew. Sacrificing blue flies or blue spiders has no special effect.
Sharp Key: Invokes the effect of 
Stitches: Creates a shadowy line similar to 
Urn of Souls: The stream of fire is replaced with a controllable 
Brimstone beam that destroys projectiles and deals more damage the longer it's active. The minimum damage is 3.37 + 0.361 × floor, and the maximum damage is 4.2 + 0.45 × floor, which it scales up to over the course of 3 seconds of continuous fire. Souls dropped from enemies are colored red, and the Urn displays a red graphic on the HUD when active.
Vade Retro: Instead of exploding on the spot, souls home in on nearby enemies and explode on contact, similarly to 
Purgatory. The souls deal 2 × Judas's damage up to 3 times and destroy projectiles.
Wavy Cap: Gives a damage bonus which lasts until the next floor equal to 0.2 × the maximum number of times Wavy Cap's effects have been stacked.
3DS The icon of The Book of Belial as seen on the lower 3DS screen.
Satan in some Jewish and Christian texts.
Spindown Dice on an item pedestal of item ID 
The Ladder, it'll skip to 
Book of Shadows.
Books 
The Binding of Isaac: Rebirth
