If used after clearing a floor, before proceeding to the next one, it guarantees a new Heart Container every floor.
Since it adds the eternal heart directly to the health bar instead of spawning it on the floor, the item is useless for Keeper and Tainted Keeper.
Synergies
Book of Virtues: Spawns white, heart-shaped wisps in the inner ring. Wisps' tears have 0.75 shot speed. The wisps' tears effectively act as though they have Sacred Heart.
Car Battery: Grants one full Heart Container upon use.
Tainted Eden: Health ups granted by eternal hearts are not rerolled when Tainted Eden takes damage.
Interactions
Tainted Magdalene: Doesn't benefit from her passive ability, giving only 1 Eternal Heart as usual.
3DS The icon of Prayer Card as seen on the lower 3DS screen
Trivia
In the Christian tradition, holy cards or prayer cards are small, devotional pictures mass-produced for the use of the faithful. They usually depict a religious scene or a saint in an image about the size of a playing card. The opposite side typically contains a prayer, some of which promise an indulgence for its recitation.
Eternal Heart upon activation.
Keeper and 
Tainted Keeper.
Book of Virtues: Spawns white, heart-shaped wisps in the inner ring. Wisps' tears have 0.75 shot speed. The wisps' tears effectively act as though they have 
Sacred Heart.
Car Battery: Grants one full Heart Container upon use.
Tainted Eden: Health ups granted by eternal hearts are not rerolled when Tainted Eden takes damage.
Tainted Magdalene: Doesn't benefit from her passive ability, giving only 1 Eternal Heart as usual.
3DS The icon of Prayer Card as seen on the lower 3DS screen
The Binding of Isaac: Rebirth
