mission from God
n. A crucially important task that must not fail; often used ironically.
But a brighter day has dawned for those seemingly on a mission from God to fatten the tax base and create jobs in Gwinnett County till the growth fills us, stills us or kills us.
—Larry Wilkerson, “Fatigued fighters of endless growth are hitting the wall,” The Atlanta Journal and Constitution, November 08, 1998
Power Play is an undertaking fraught with hazard, says Laurin and his partner Glenn Davis, precisely because Canadians have such intense feelings about the game….

"We keep saying, 'We're on a mission from God: we're doing a hockey show for Canada. We'd better get it right,"' says Laurin.
—Tony Atherton, “A chastening look at modern hockey,” The Ottawa Citizen, September 26, 1998
1984 (earliest)
But Cornbread got the sweet last licks in Tuesday night's finale. He did a double number on Worthy, outplaying him and out-talking him. If he hadn't succeeded at the former, he would have seemed an awful fool for the latter. His jive talk — "You can't guard me" — would have been idiotic. Instead, it proved prophetic.

Never in doubt, insisted Maxwell.

"I was on a mission from God tonight," he said after the game in the champions' insane locker room.
—Bob Rubin, “Cornbread gets last, best licks,” The Miami Herald, June 14, 1984
The popularity of this phrase is almost certainly due to its frequent use in the popular movie The Blues Brothers (1980):

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