A Southern California temple is trying to shame Mel Gibson into undoing a mess it created by taking out a jumbo loan on its synagogue -- a loan it can't afford to pay back.

Congregation Beth Shalom, located in Corona, sent Mel a letter -- obtained by TMZ -- in which it is requesting big money from the actor to prevent foreclosure.

Now here's the shameful part. In a move resembling a dude accosting you for money at a gas station, the letter reads in part, "Our proposal to you, Mr. Gibson, is since you have been cited as an Anti-Semitic, and have denied those allegations, what better way to prove to all your fans and the nay Sayers -- than to endorse and help raise funds for our cause -- SOS, Save Our Synagogue."

We know Gibson has contributed a fortune to a bunch of L.A. charities, including $5 million to Cedars-Sinai Medical Center -- a Jewish hospital.

Mel's rep tells TMZ ... he knows nothing about the request but wishes the temple all the best.

A rep for the temple tells TMZ, "We weren't trying to shame Mel Gibson into giving us money. We are approaching many celebrities and people of the community as well. We thought because of his situation he would be very much interested in supporting our cause."