The Bible says it is better to give than to receive. So Rev. Filbert Moore will hand over the keys to his $10,000 sports car to anyone willing to buy his $145,000 five-bedroom house with swimming pool in Gaithersburg.

Rev. Moore's offer, however, is not born totally of benevolence; instead, he is trying desperately to sell his brick colonial home on Goshen Road in the midst of one of the nation's worst housing slumps.

For the past 33 months, researchers at the National Association of Realtors have flinched at recurring reports of a declining housing market. "It's kind of scary. The national scene is pretty grim, pretty grim," said NAR spokesman Bill Ellingsworth.

The housing industry is struggling against 17 percent interest rates and reluctant buyers who can't afford mushrooming mortgages. The current downward trend started in October 1979 with former President Carter's economic recovery program, which spurred the Federal Reserve Board's tight-money policy. Since then, said Ellingsworth, the real estate industry has lost $162 billion that it would have earned in a healthier economy. "That's the real shocker. It's a real drain on the economy."

Moreover, he said, nationwide existing housing sales are running 41 1/2 percent below the industry's projections for the past three years.

Sales of existing single-family homes fell 5.2 percent in July, reversing an upward shift in the previous month, the NAR reported.

The picture for the sale of newly built homes is not much brighter. The Bureau of Census recently released a gloomy report that shows sales have fallen steadily since 1977, when 819,000 new homes were sold. That figure dropped to 817,000 in 1978, 709,000 in 1979 and 530,000 in 1980.This year's total is not expected to exceed 500,000.

The government is scheduled to announce July sales of new single-family homes Monday. In June, sales of these homes fell 17.2 percent, to the lowest level since the bottom of the 1980 recession. That same month, however, the NAR reported sales of existing single-family homes rose 6 percent.

This week, Jack Carlson, the National Association of realtors' chief economist, said existing houses were selling at a seasonally adjusted rate of 2.52 million units. "If this trend continues, the sales total for 1981 will fall below last year's low total of 2.8 million resale transactions and would be 32 percent below the pre-recession levels of 1978 and 1979," he said.

In Washington, the nation's third most expensive housing market behind San Francisco and Los Angeles, residential sales for July were off 10.9 percent compared with July 1980, according to Lusk's District of Columbia Real Estate Guide. Despite the decline, Executive Vice President Rufus Lusk III said, Realtors have no reason to "start jumping out of windows."

Ironically, condominium sales are up sharply in Washington over last year, although the average sales price has increased 22.7 percent. The average condo price in 1980, according to Lusk, was $70,535, compared with $81,965 in 1981.

However, the city's residential sales in the first six months of each year have plummeted from 2,909 in 1979 to 2,110 in 1980 and 1,808 in 1981 -- a 62 1/2 percent decline.

"New condo sales are coming on strong because a lot of new condos are coming on the market with discounts to tenants, and they are less expensive [than single-family homes]," said Lusk. "Condos are very popular and fit the smaller family size in the District."

In Northern Virginia, in the first half of 1981, sales are 9 1/2 percent below 1980 sales from January to July, according to Joe Hayden, spokesman for the Northern Virginia Board of Realtors. However, the July sales alone were 37 percent below July of last year as prices increased 14 1/2 percent on the average single-family home, rising from $90,613 to $103,814.

Hayden said the market is plagued by buyers who feel, "'Gladys, this is not the time.' But there really are people buying and selling houses. High interest rates are one of the finest tax shelters in the world."

Sales in Prince George's County during the first seven months of 1981 and 8 percent behind the comparable period last year. In Montgomery County, the news is worse. Residential sales are off 35 percent compared with 1980. "Obviously, this is a serious problem. There's no question about it," said Paul Fowler, executive vice president of the Prince George's Board of Realtors.

Most Realtors agree that the sole remedy for their ills lies in a stronger economy. Ellingsworth said President Reagan's Economic Recovery Tax Act of 1981 should give the industry a lift with its emphasis on a balanced budget by 1984 and the sale of all-saver's certificates in October.

Seventy-five percent of the profits from the sale of the certificates, according to Ellingsworth, are earmarked for residential lending and agricultural loans, which he predicts will be offered at two points below current mortgage rates. In addition, the National Association of Realtors is asking the Federal Reserve Board to cut the prime lending rate by at least one point.

If that program doesn't work, Gladys, it may not be the time.