The spawned door will not disappear when leaving and re-entering the boss room (except in Greed Mode).
Eucharist applies two effects. Like Goat Head, it fixes the Devil Room chance at 100%, which forces an Angel/Devil Room to spawn whenever it can; it also fixes the Angel Room chance at 100%, so the game always chooses an Angel door over a Devil door.
Now instead fixes the Angel Room chance at 100%, and the Devil Room chance at 0%.
If the Devil Room has been visited before picking up Eucharist, an Angel Room will not replace it, despite the Angel Room chance displaying 100%.
Duality: Both Angel and Devil Room doors spawn after every boss, and the selected door will never disappear.
This is the only way to encounter Devil deals while possessing Eucharist.
Filigree Feather: Due to Angel Rooms being guaranteed, a large influx of items can be obtained over time.
A eucharist is a Christian rite in which bread and wine are consecrated into the body and blood of Jesus Christ on an altar and consumed thereafter.
Afterbirth+ item concepts, including for Eucharist.
If Isaac touches a Red Chest before picking up Eucharist, the game will determine whether or not he gets a Devil/Angel room that floor even if he doesn't teleport, nullifying Eucharist's effect if a devil deal is chosen.