DeNiro as Jewish - why?

But the issue of Judaism doesn't get raised in the movie at all. Why do they even make him Jewish?

In The Godfather, we are dealing with the Sicilian mob in New York, and they make a huge deal out them being Sicilian - we get the backstory from Sicily, they speak Italian on occasions, and they travel to Sicily at least four times. In this movie, we are told that he is Jewish, and that's it. It seems kind of a pointless notion to point out that a character is Jewish, if you are not going to use that fact for something in the movie at all.

There was nothing wrong with his acting; I agree completely that he plays well, in this one as in nearly every movie he has been in. I was just wondering why he seems to get typecast as Jewish gangsters, when he has nothing to do with Judaism. Once Upon A Time In America also has him portraying a Jewish mobster.

As real as the Highlander-thread questioning why a Frenchman was cast as a Scotsman and a Scotsman as an Egyptian. Or, for that matter, why Brad Pitt was cast to play a Jewish character from the American South (???) in Inglourious Basterds.

It is not a criticism of DeNiro's acting, only a matter of lacking authenticity, when actors are cast to play ethnicities that they clearly not are themselves. Isn't that a reasonable question to raise?

The real life man Ace was based on, Frank Rosenthal, was Jewish. Scorsese didn't just make up a Jewish character so he could work slurs into the film.