mission from God
n. A crucially important task that must not fail; often used ironically.

Example Citation:
"But amid the soul-searching, it is unlikely that Nicholson, himself the object of an intraparty challenge this winter, can keep in check the passions and egos of politicians, some of whom see themselves on a mission from God to save America from itself."
—Scott Shepard, "GOP fearful in fighting will cost elections," Austin American-Statesman

The popularity of this phrase is almost certainly due to its frequent use in the popular movie The Blues Brothers (1980):
    Jake Blues (John Belushi): "First you trade the Cadillac 
    for a microphone. Then you lie to me about the band. 
    Now you're gonna put me right back in the joint!" 

    Elwood Blues (Dan Aykroyd): "They're not gonna catch us. 
    We're on a mission from God."

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