The greatest grift. You promise your whole life/time/talents and tons of money, to the church itself. The reward is only given after death, but not by the church (which you gave everything to). The church gets it all, and gives you nothing. You just have to hope that lies are true and that God really will reward you.
It justifies absolute misery here on earth. Earthly suffering doesn't matter, since the afterlife is all that matters.