Completion marks are earned by completing specific objectives in The Binding of Isaac: Rebirth. When completing these objectives, the post-it note on the character select screen fills up with Completion Marks. If objectives are completed in Normal mode, the completion marks are solid black. If objectives are completed in Hard mode, the completion marks are outlined in red. Earning a completion mark in Hard mode will unlock marks for both Normal and Hard modes, even if the prior wasn't unlocked.
In Rebirth, Mom's Heart / It Lives! is the only boss with a character-specific co-op baby unlock if you beat it in Hard mode instead of Normal mode, with one exception - 
Godhead, which is unlocked by obtaining every Hard mode completion mark with The Lost. as such, it is not necessary to earn every Hard mode completion mark to achieve Platinum God, only every Hard mode Heart mark, with The Lost as an exception. In Afterbirth and onward, every other regular character has a co-op baby that can be unlocked for earning all of their completion marks on Hard mode. There are a few additional rewards for completing many or all completion marks.
In Rebirth, there are 6 unique completion marks, and each one comes in two varieties, adding up to 12 marks in total. Afterbirth introduced 3 more, bringing the total to 17, as Greed mode only has one mark. Afterbirth+ introduced 1 more, as well as an additional mark for Greedier mode, bringing the final total to 20. Repentance introduced 2 more, bringing the final total to 24. Each mark has a character-specific unlockable secret associated with it. There are 6 secrets per character in the base game, while there are 10 in Afterbirth, 12 in Afterbirth+ and 14 in Repentance.
Tainted characters have different requirements for unlocking secrets. One secret each is unlocked for beating timed challenges (Boss Rush and Hush) and main path bosses (Isaac, ???, Satan, and The Lamb). There are no secrets for beating normal Greed mode, as well as no character-specific unlocks for earning all completion marks on Hard mode, bringing the secret total down to 7 per character.
Notes
As Greed mode is a unique mode separate from Normal or Hard mode, it always appears on the completion note with the red outline otherwise found on Hard mode marks.
This symbol is used to indicate completion of Greedier mode, and the Greed mode symbol is changed to be one without a red outline.
- While it is possible to fight Mega Satan in the base game, completion marks for defeating him and their respective unlocks are only available in the subsequent DLC.
- In the base game, Godhead is unlocked by defeating
Mom's Heart, 
Isaac, 
???, 
Satan, 
The Lamb, and 
Boss Rush on Hard mode as 
The Lost.
- It is necessary to additionally defeat
Hush, 
Mega Satan, and 
Ultra Greed.
- It is also necessary to additionally defeat
Ultra Greedier and 
Delirium. This also applies to all other unlocks requiring beating all bosses on Hard mode.
- It is also necessary to additionally defeat [[Mother}} and
The Beast. This also applies to all other unlocks requiring beating all bosses on Hard mode.
- Some items and trinkets transform the current character into other characters. All completion marks earned in that run from the moment of death onwards will count towards the new character, not the old.
Judas' Shadow transforms the current character into 
Black Judas upon death. Black Judas is considered the same character as 
Judas by the game and can earn all of Judas' completion marks.
Ankh transforms the current character into 
??? upon death, and 
Broken Ankh has a 22.22% chance to transform the current character into ??? upon death.
Lazarus' Rags transforms the current character into 
Lazarus Risen upon death. Lazarus Risen is considered the same character as 
Lazarus by the game, and can earn all of Lazarus' completion marks. The only thing Lazarus Risen can't do is unlock 
Bethany.-
Missing Poster transforms the current character into The Lost upon death if he's already unlocked.
Clicker transforms the current character into a random character upon use.
- Only transforms into a character of the current side (i.e. Tainteds into other tainteds, non-tainteds into other non-tainteds)
- With Clicker and a reliable method of reaching the
I AM ERROR room (such as 
Teleport 2.0or 
Undefined, held with a 
Schoolbag), 
The Void can be repeated endlessly and cleared with every playable character in one run.
- By using a
Forget Me Now (and a method to duplicate it endlessly) this can be applied to any floor and boss in the game, allowing any completion mark to be obtained with every character in one run. A 
Crooked Penny combined with a 
Glowing Hourglass to rewind failures is a good approach. 
Moving Box will also be necessary to bring all of the required items with you on each reset.
- Boss Rushrequires the additional duplication of a
Mama Mega! to repeat endlessly.
- Mega Satan can only be repeated with the addition of other unlimited items that can unlock the door, such as a duplicated
Get out of Jail Free Card, 
Dad's Key, or 
Mr. ME!, because the key disappears after a single-use. Mega Satan also has a 50% chance of automatically ending the run upon defeat, so it is only recommended to focus on that completion mark after finishing all other possible marks as the desired characters.
- Completion marks for Satan and The Lamb are still mutually exclusive with completion marks for Isaac and ???, as well as with The Beast and
Mother. This means a minimum of four runs are required to obtain every character's completion marks using this strategy.
- This strategy does not work for Greed mode completion marks, as Moving Box is not in any item pools (disregarding an
Eden spawn and 
Spindown Dice).
- Up to 3 Greed mode completion marks can be obtained in a single run using 2 or more items that revive you as a different character – alongside 2 Forget Me Nows.
- Getting all these items in a single run usually requires the use of a strategy to create infinite items and exhaust the item pool, like
Blank Card / 
Clear Rune combined with 
Jera. While this can be time-consuming to achieve in a single run, it can save you a great deal of time and additional runs if you are aiming for 100% completion.
- Substituting Clicker with items that revive you as another character upon death will allow you to get the extra completion marks for The Lost, ???, Lazarus, and Judas, but will not allow for completion with every character as the Clicker does.
- Beating the game with multiple players will count towards each of the players' completion marks.
Online (open beta) added a new Delirium icon and also blue and purple completion marks (only shown online). Blue is for completion marks that were completed in online mode only, and purple is for the completion marks completed in both hard and online mode. (online is hard mode only)
Completing normal/greed mode runs online turns the completion marks blue, and completing hard/greedier runs turn the completion marks purple.
Gallery
A flowchart on the routes to each completion mark
All completion marks in Online mode
Trivia
- Crumped | Torn up paper might only go further with the notion of Isaac being fed up with his own thoughts.
Isaac
Magdalene
Cain
Eve
Samson
Azazel
Lilith
Keeper
Apollyon
The Forgotten
Jacob and Esau
