The eternal heart is added directly to Isaac's heart containers, as opposed to appearing as a pickup.
The effect will also be triggered when the floor is reset (for example, with Forget Me Now).
Eden will not be given the eternal heart on the first floor if they start with this trinket.
When held as The Forgotten, the eternal heart will be given to whichever character is being controlled when entering the next floor.
When going though the Ascent, eternal hearts will be given for each floor traveled up.
Glowing Hour Glass: Eternal hearts will persist if the item is immediately used on the next floor, allowing for another eternal heart for free.
Mom's Box: Grants two eternal hearts at the start of each floor, which is equivalent to gaining a full heart container every floor.
Maggy's Faith is most likely a reference to Mary Magdalene's faith in Jesus. Mary was said to be one Jesus trusted most and she stayed with Him past the abandonment of His disciples, as well as being the first person He showed Himself to after His resurrection.
"Faith's reward" may constitute a contraction of the adage "Faith is its own reward", although use of the word 'faith' in the game is conflicted, as it also used in the quote for the trinket "Rosary Bead", in reference to Angel Rooms.