What was going on when I was born?

Three Lutheran denominations voted today to form a new church for 5.5 million members that will be the third largest Protestant denomination in the country. Dr. Martin Marty, a church historian at the University of Chicago, said that the action marked a 'decisive point' in American church history and that there had never before been such a merger in an American denomination. The merger brings together three of the five major Lutheran denominations in the United States. The denominations have a common theology but diverged according to the countries from which Lutheran immigrants came here.

Last Thursday night, as police officers and other emergency personnel watched from below, Edward McGuire grabbed and wrestled to safety an 18-year-old man threatening to jump from the roof of a six-story apartment building here. Many of those who watched the ordeal and others who heard about it are calling Mr. McGuire's action ''heroic.'' But the Larchmont police are calling it an ''obstruction of governmental administration'' - a misdemeanor punishable by one year in jail or a fine of $1,000, or both.

Cautious American consumers took on only $570 million more in installment debt than they paid off in July, the smallest debt increase since February, Government figures showed today. The Federal Reserve Board report covered the first month after the Reagan Administration's big tax-rate cut took effect on July 1. Businessmen and Government officials have been hoping the cut would help generate a surge in consumer spending that would pull the economy out of a year-long recession. But most economists said they did not expect big spending increases to show up immediately.

The stock market tried to resume its late summer rally yesterday after Tuesday's sharp sell-off but managed to close with only small gains in very heavy trading. The Dow Jones industrial average finished the session up 1.47 points, to 915.75. On Tuesday, profit taking sent the Dow down 10.85 points after the key indicator had soared 148 points, or 19 percent, in the previous 16 sessions. Stock exchanges were closed Monday in observance of Labor Day.

Chemical Bank, in one of the first announcements of its kind, said yesterday that it would introduce a service that would allow its customers in the New York area to bank at home using personal computers. Such home electronic banking has been the subject of numerous market tests, but Chemical, the nation's sixth-largest bank, would be the first of the nation's largest banks to offer such a service commercially. Customers using the system would be able to do such things as keep track of their checking accounts and home budget, transfer money from one account to another and pay bills electronically by typing commands on the computer keyboard. Mark Plakias, research director for Link Resources Corporation, a New York market research firm, said that as home banking, shopping and other electronic services achieved commercial status, other companies were likely to follow Chemical. Its move ''indicates the consumer is much more ready to use this kind of a product than most people imagine,'' he said.

If you ask Samuel I. Schwartz about ''hot spots'' around town, he will not tell you where you can go to boogie all night. Hot spots to Mr. Schwartz are places where motorists and car engines get overheated in heavy New York City traffic. Far and away the hottest spot these days is Herald Square, at 34th Street and the Avenue of the Americas. Traffic moves there at an average speed of 2.2 miles an hour, or to put it another way, you can walk nearly two blocks before the traffic moves one.

B'nai B'rith, the American Jewish service organization, praised President Reagan's plan for the Middle East today and said it was ''worthy of consideration.'' Jack J. Spitzer, the organization's international president, said in a statement that although the group had specific questions and problems with Mr. Reagan's proposals, ''we believe they are presented in a sincere spirit of finding common ground for involving all parties to the conflict in the negotiating process and giving fresh momentum to the search for Middle East peace.'' Since Mr. Reagan's speech on the Middle East last week, Jewish and Arab-American groups have been discussing the plan at some length. For the Jewish groups, the problem has been complicated by the Israeli Government's rejection of the plan.

A gift of $430,000 for a training program to improve the leadership skills of principals in the city's 110 high schools will be announced today by the Chase Manhattan Bank. The gift is to be made to the New York City public schools and the New York Alliance for the Public Schools, which helped shape the idea for the program. The alliance, organized by New York University, is composed of representatives from the school system, higher education and the business community. The money from the bank, one of the largest gifts ever received by the school system from a private donor, will be used over two years to pay for seminars, conferences and other activities designed to enable the principals to learn more about how to do their jobs effectively, particularly in planning and implementing curriculums.

''All I want to do is raise my right hand to Mayor Koch and take that oath to be a New York City firefighter.'' - Katrina Cannon. (A1:3.)

IN the late 1960's and early 70's the clean, open spaces, the white walls, sleek built-ins and industrial carpeting of minimalist design were the sine qua non of contemporary interiors. Then postmodernism came along, with its pastel hues and its vocabulary of classical architecture. Now, with postmodernism losing much of its novelty, some architects and designers are bringing back minimalist ideas, but in a slightly altered form. What is emerging could very well be a new style, one that might be called postminimal. Here minimalist principles of spareness are enriched by the use of warm woods, earthy materials and a rustic simplicity.

Foreign Minister Yitzhak Shamir said today that if the Reagan plan for the occupied West Bank and Gaza Strip had been put forth as the American position four years ago Israel would never have signed the Camp David accords that led to peace with Egypt. Mr. Shamir said President Reagan's plan had disturbed the delicate balance of the peace process, undermining the accords and damaging the prospects for their success. The Foreign Minister, who opposed the Camp David framework when it was worked out in 1978, spoke on behalf of the Israeli Government during a debate in Parliament on the United States proposals. Begin Backed in Parliament Parliament then voted 50 to 36 to endorse the Begin Government's opposition to the Reagan plan, which calls for Palestinian rule in the West Bank and Gaza in association with Jordan. The plan also urges a freeze on Israeli settlements in the occupied territories.

CITY gardeners sometimes forget that when summer flowers vanish, terraces or yards can be dreary and gray under winter skies. The best way to prevent this is to plant several of the slower-growing evergreen shrubs and trees that will keep their leaves through the colder months. Quite a few of these have a proven city toughness, and now is the time to add them. Yard owners can plant until the first frost (late October or early November) while terrace gardeners and those in high, windy spots should complete their planting by the end of this month.