How To Unlock Every Main Boss

• Defeat Mom 1 time (without mods or the debug console enabled) to unlock the Womb floors and Mom's Heart. Achievements can then be unlocked even with mods and the debug console enabled. I highly recommend all players use the External Item Descriptions mod so you don't have to manually look up the effect of every item you find.
  
• Defeat Satan 5 times to unlock The Negative and Isaac 5 times to unlock The Polaroid. If you get a devil / angel deal from the Mom's Heart fight, you can go into the devil / angel room and progress to Sheol / Cathedral to fight Satan / Isaac.
  
• Defeating Mom's Heart 11 times will turn Mom's Heart into It Lives. Defeating It Lives will always spawn an entrance to both Sheol and Cathedral without needing to get a devil / angel deal.
  
• Once The Negative / Polaroid are unlocked, you can defeat Satan while holding The Negative to progress to the Dark Room and fight The Lamb. Alternatively you can defeat Isaac while holding The Polaroid to progress to the Chest and fight ???.
  
• Defeating The Lamb or ??? once will make it possible to blow up the angel statues that appear in angel rooms. This causes the statue to turn into a boss which you can kill for a Key Piece. Do this twice in one run to get a full Key, then you can use it to unlock the Mega Satan door in the Dark Room or the Chest.
  
• Defeat Mom's Heart 10 times to unlock the Blue Womb floor, where you can fight Hush. The Blue Womb can be found by going through a large door that is spawned by defeating Mom's Heart before the 30 minute mark in any run.
  
• Defeat Hush 1 time to unlock the Void floor, where you can fight Delirium. You can get to the Void by going through a portal that is sometimes spawned when defeating various late game bosses. Hush is guaranteed to spawn this portal if you defeat him (check the top right corner of his boss room after defeating him).
  
• Defeat Hush 3 times to unlock the alt floors, which eventually lead you to Mother. The first two normal floors of any run will now have a locked door (requires a key) in the boss room which leads to the first alt floor, Downpour. The 3rd and 4th normal floors (and Downpour II) will have a barricaded door (requires 2 bombs) in the boss room which leads to the second alt floor, Mines.

Downpour II has a white fireplace somewhere on the floor, and a room with a mirror in it. Touch the white fire to temporarily turn into The Lost, then walk thru the mirror to access the mirror world. The mirror world Item Room will have a knife piece. Mines II has 3 yellow buttons somewhere on the floor, and a room with a minecart in it. Press the buttons and then ride the minecart. Walk to the end of the path to get the 2nd knife piece.

The Mines II boss room will have a special door which you must pay two hearts to open, and leads to the 3rd alt floor, Mausoleum. Defeat the harder version of Mom on Mausoleum II, use your completed Knife on the red door in the boss room, and then defeat the harder version of Mom's Heart. Then go back to Mom's boss room and go down the hole to the 4th alt floor, Corpse. Finally, you can fight Mother at the end of Corpse II.

• Defeat Mother 1 time to unlock a Strange Door in the starting room on Depths II / Necropolis II (floor 6), which eventually leads to The Beast. Somewhere on floor 6 will be a skull rock with an x mark on it, which can be destroyed to get a Fool card. After defeating Mom use the Fool card to get back to the Strange Door, which you can now open by giving it the Polaroid or Negative. Go inside and then down the hole to the Mausoleum.

The boss room of this version of Mausoleum will not have a boss, but instead an item called Dad's Note. Once you pick it up, you will ascend back up through the floors you previously cleared on the run. The floors are smaller than normal, but don't have any special rooms except item rooms (which will not have new items). Once you make it back to the starting room on floor 1, you can go into the light to get to Home. Sleep in Mom's bed on the left side of the floor, then go back into the living room to start the final sequence of boss fights, ending with The Beast.

The content of this guide comes from a post by one of the reddit users named GoldPhos. I'll leave a link below, he deserves thanks <3

• https://www.reddit.com/r/bindingofisaac/comments/tkjcgv/repentance_progression_guide_unlocking_all