Gospel Duo Mary Mary -- Accused of Stiffing Manager ... Making God Sad

What would Jesus say? Gospel duo Mary Mary enlisted a manager to revamp their careers, then screwed him over as soon as they made it big again ... at least according to a new lawsuit.

Maximum Artist Group -- owned by manager Mitchell Solarek -- filed a federal lawsuit in Tennessee against Mary Mary, aka Erica Campbell and Tina Campbell.

In docs, Mitchell says he inked a deal with the duo in 2011 and helped "reignite" their career ... securing deals worth over $3 million, including their WeTV reality show.

In return, Mitchell says the Christian singers went all Judas on him -- publicly firing him on their TV show, and withholding more than $75,000 in commissions.

Mitchell is suing for all that dough, and the loss of future commissions from music, merchandising, books and sponsorships.

Now here's Mitch before the axe came down ...