This is very very loosely the plot of The Sparrow by Mary Doria Russell. They weren’t sent to necessarily convert the aliens, but the Catholic Church were the only ones willing to bankroll a trip that would take at least 60 years and have no possible ROI.
It definitely does not go as planned and, maybe unsurprisingly to modern readers, ends with the Catholic Church being cast as a bunch of perverts in the public eye.
I ate the onion. It would have been awesome, but since lynchings were still so common after the end of segregation I guess it would have been a risky profession.