What was going on when I was born?

The Government index that is intended to forecast upturns and downturns in the economy fell 2.7 percent in September, the Commerce Department reported today. The drop in the index of leading economic indicators was the largest monthly decline since last year's recession and was more evidence that the nation's economy is again in a recession. The index measures business activity that leads the general trend of the business cycle by several weeks. Last month's decline in the index was the fourth in the last five months and the largest since a 4 percent drop in April 1980, near the low point of last year's short but steep recession. The index fell five-tenths of 1 percent in August after rising three-tenths of 1 percent in July.

Not since the Penn Central railroad tried to put a skyscraper atop Grand Central Terminal has a land-use issue in midtown Manhattan ignited such passions as the proposal to sell off some of St. Bartholomew's Episcopal Church on Park Avenue for the construction of an office tower. Indeed, if the Grand Central controversy, which spanned the decade from 1968, when the skyscraper was first proposed, to 1978, when the United States Supreme Court put it finally to rest, could be said to have been the major landmarks preservation battle of the 1970's, this one seems to be shaping up to play the same role in the 1980's. The reason is simple. St. Bartholomew's, the Byzantine church built in 1919 to the designs of Bertram Goodhue, sits on the east blockfront of Park Avenue between 50th and 51st Streets. It is on the last piece of land on the city's premier commercial boulevard not now occupied by commercial real estate.

Law enforcement officials said last night that evidence had been uncovered supporting earlier suspicions that the gang of Black Liberation Army and Weather Underground members accused of the Brink's armored-car robbery in Rockland County last week was also responsible for an earlier Brink's robbery and slaying in the Bronx. The officials said that agents of the Federal Bureau of Investigation had found a fingerprint of David J. Gilbert, one of those charged with the Rockland crimes, on the rental agreement for a van that was used in the Bronx robbery. One Brink's guard was killed and another was wounded in the Bronx holdup on June 2. The robbers got away with $200,000. The officials said that witnesses to the Rockland robbery in which three persons were killed were shown composite sketches of the suspects in the Bronx crime and that they said they believed they recognized them as some of the Rockland suspects who are still at large.

President Reagan followed up his victory on the sale of Awacs planes to Saudi Arabia by telling Israel that the United States is ''fully committed'' to helping Israel retain ''its military and technological edge'' over the Arabs. Dean Fischer, the State Department spokesman, said today that Mr.Reagan sent a letter with that assurance to Prime Minister Menachem Begin last night. He said the President had also sent a message to King Khalid of Saudi Arabia urging ''close cooperation'' between Washington and Riyadh in meeting ''our mutual responsibilities with respect to the pursuit of peace, security and prosperity'' for the Middle East and the rest of the world. The texts of the letters have not been released.

The president and the executive vice president of the Nassau County Community College were arrested today on indictments charging them with grand larceny, conspiracy and official misconduct. A professor at the 23,000-student school, a former vice president and a public-relations officer were also arrested on charges growing out of a county grand jury investigation into reports of conflict of interest and other improprieties. The school is the largest community college in New York State. Charged were Dr. Robert Gwydir, the president of the college since 1979 and a member of the staff since the school was founded 22 years ago, and Dr.Albert Donor, the executive vice president.

In a move designed to shore up the state's financially troubled savings banks, the state Senate last night passed a bill providing regulatory and tax relief to the state's thrift institutions. And in a highly unusual move, the Legislature voted to override Governor Carey's veto of a bill granting a preference to American structural or reinforcing steel in state construction projects. Mr. Carey had argued that the bill would inhibit state efforts to attract foreign industry. But the proponents who pushed the measure onto the state's statute books said it would help save jobs in the domestic steel industry. In 1976, Mr. Carey became the first New York governor in more than 100 years to have a veto overidden, and since then the Legislature overrode him on policy bills twice.

The prime lending rate was lowered one-half percentage point, to 17 1/2 percent, yesterday by the Continental Illinois National Bank and Trust Company, the nation's seventh-largest commercial bank, but no other major bank followed its lead. Credit analysts were divided on whether Continental's move would be followed soon by other banks, which lowered their primes to 18 percent in mid-October.

Goldman, Sachs & Company, one of the world's leading investment banking houses, said yesterday that it had acquired J. Aron & Company, the nation's biggest supplier of green coffee beans to roasters and a major factor in the international precious and base metals trade. Officers of Goldman, Sachs and J. Aron, both private partnerships, declined to put a dollar value on the merger. However, industry sources said the price was slightly more than $100 million. The capital of Goldman, Sachs, $239 million as of last June 30, makes it the largest financial institution operating as a partnership. The industry sources estimated the revenues of J. Aron at more than $1 billion last year.

''The groundswell of conservatism evidenced by the 1980 election makes this an especially appropriate time to urge upon the courts more principled bases that would diminish judicial activism.'' - Attorney General William French Smith. (A1:1.)

The 13 members of OPEC agreed today on a unified price for crude oil that is expected to push up the retail cost of gasoline and heating oil in the United States by 3 cents or more a gallon. The new common OPEC base price was set at $34 a barrel - an agreement that required Saudi Arabia to raise its OPEC communique text, page D13. base price by $2 a barrel while the 12 other members cut theirs by the same amount. The agreement, reached at a special one-day meeting of the Organization of Petroleum Exporting Countries, represented the first time that OPEC had officially decided to lower prices charged by most of its members.

A dispatch from Paris on Monday in- correctly listed speakers who ad- dressed a rally opposing arms in- creases.

SPAIN PREPARES TO JOIN NATO: King Juan Carlos, left, with his son, Prince Felipe, arriving at a military practice zone near the Mediterranean. Yesterday, the Con-gress of Deputies, the lower house of Parliament, approved a plan to join the Atlantic alliance. The Senate is expected to approve the proposal next month. Page A11.