Emissary of Gaabriel

Flying, lifelink.
Champion a creature (When this enters the battlefield, sacrifice it unless you exile another creature you control. When this leaves the battlefield, that card returns to the battlefield.)
Emissary of Gaabriel gets +X/+X, where X is the championed creature's devotion to white (Each {w} in the mana cost of the championed creature counts toward its devotion to white.)