Not only was it protested by Jewish groups- according to Robert Sellers, author of a book about George Harrison's production company which made Life of Brian, the very first public protest of the movie was in an article in Variety only two days after the movie first hit theaters, and it was by a rabbi named Abraham Hecht, representing an Orthodox Jewish group called the Rabbinical Alliance of America (of which he was president). The group was known (and still is) as a conservative (with a small c, not to be confused with the Conservative movement in Judaism) faction of rabbis, which was at the time led by Hecht.

Unfortunately, I have been unable to track down the original article in Variety in which Hecht gave his remarks, but it was cited in other news coverage. A contemporary article in the New York Times (Aug 28 1979) says that Hecht called the movie “blasphemous,” “grievously insulting” and “a crime against religion.” According to the same article, he stated that “The ‘Life of Brian’ is a vicious attack upon Judaism and the Bible and a cruel mockery of the religious feelings of Christians as well,” and that "this film is so grievously insulting that we are genuinely concerned that its continued showing could result in serious violence... [people may be] moved to violence because it's such a bad movie.” A 2003 article in the Guardian adds that Hecht said of the movie "never have we come across such a foul, disgusting, blasphemous film before" and called it "produced in hell."

Now, the New York Times article is titled "Three Jewish Groups Condemn 'Monty Python's Life of Brian'"- of course, one of the three groups was the Rabbinical Alliance of America, but he also claimed to represent two other groups, the Union of Orthodox Rabbis of the United States and Canada (also known as the Agudath haRabanim, NOT to be confused with the modern-day Orthodox Union) and the Rabbinical Council of Syrian and Near Eastern Sephardic Communities of America. According to Hecht, the three groups represented over 1000 rabbis.

It's worth bearing in mind, though, that these three groups represented a very small percentage of American Jewry, and I've found little evidence of any particular call to protest beyond the relatively basic quotation in newspapers. I also believe that the only reason for the involvement of the Rabbinical Council of Syrian and Near Eastern Sephardic Communities of America was that at the time, Hecht was the rabbi of one of the largest Sephardic synagogues in the US, Congregation Shaare Zion in Brooklyn. I have seen absolutely no evidence that any grassroots initiative to protest the film existed, and also find it worthy of note that the Agudath HaRabanim largely consisted of rabbis who had negative views toward their constituents watching any films, "blasphemous" or not. I can't escape the conclusion, therefore, that this was a personal initiative of Hecht's and that he harnessed the name power of the multiple organizations with which he was affiliated to make this happen.

Now, interestingly, Monty Python themselves, in The Pythons Autobiography by The Pythons, mention that they were surprised to get news of protests by "1000 rabbis" (clearly just a report of Hecht's article) because they'd taken out the scene that they thought would be actually controversial- a scene portraying a Jewish fascist movement which they apparently now wish they'd kept in. They also mention an "Association of New York Rabbis" which protested the use of a prayer shawl (tallit) in the stoning sequence, but I have absolutely no idea to what they are referring, especially as there is no, and as far as I can tell has never been, an "Association of New York Rabbis." (There is the New York Board of Rabbis, but I can't find any record of them saying a word about Life of Brian, though that may have happened behind the scenes...) Either way, when Monty Python members report SEEING protests of the film, they are generally of Christian protesters (such as Michael Palin reporting seeing "nuns with banners"). It is entirely possible that there were some rabbis who were among the protesters, but it was by no means a large movement of "1000 rabbis."