Isaac gains a pulsing blue aura that repels enemies and projectiles.
The power of the repel force alternates in strength at random, sometimes strong enough to counter a projectile well before it hits Isaac, sometimes only strong enough to slow it down.
Ground-based enemies will overcome the counterforce easier than flying enemies and projectiles.
Notes
A stronger effect of this item is given temporarily by I - The Magician?.
When playing as Tainted Forgotten, this item works only on the Forgotten's body and not the soul.
When playing as Tainted Jacob, Dark Esau is unaffected by this item's repelling properties.
The designs of these items appear to be that of Triquetra, a Christian symbol of the Trinity.
The item's description, along with those of The Body and The Mind, are references to the three attributes commonly ascribed to God in Christian theology: omniscience, omnipotence, and omnipresence. For The Soul, it is omnipotence, the ability to do anything, which follows naturally from the ability to "be all".
Soul Hearts.
A stronger effect of this item is given temporarily by 
I - The Magician?.
Tainted Forgotten, this item works only on the Forgotten's body and not the soul.
Tainted Jacob, Dark Esau is unaffected by this item's repelling properties.
It Lives.
The Body and 
The Mind, are references to the Three Primes of Alchemy.
The Lost Unlocks 
The Lamb Unlocks 
The Binding of Isaac: Rebirth
