Even though have been No. 1 for years, Southern Baptists will continue to try harder.

That was the message that came through the speeches, sermons and programs of the nation's largest Protestant communion in its annual convention here.

If there was one phrase that was heard more often during the three-day convention than the genial "Hi, y'all," it was the denomination's theme for the years ahead, "Bold Mission Thrust."

Translated, that means Southern Baptists are redoubling their missionary efforts to proclaim the gospel to the world by the year 2000. Every agency, and board within the convention, from the pension board to the Radio and Television Commission was admonished to gear its program to that goal.

To tool up for that emphasis, slated to get underway next year, the convention voted a record budget of $75 million.

Southern Baptists with 13 million members have long since burst the regional confines suggested by their name and now have churches in virtually every state. Long years of missionary endeavor abroad have brought more than a million additional members into Baptist-related chruches in 90 countries.

The Bold Mission Thrust proposal endorsed by the Atlanta convention - the largest in the denomination's history - calls for adding 1,000 more career missionaries to the current force of 2,776.

In addition, plans calls for 5,000 volunteers for a two- or three-year hitch in the denomination's newly established Mission Service Corps, modeled in part after the Peace Corps.

Mission programs cost money, and the kind of expansion the Southern Baptists are talking about is not financed by pennies from Sunday school classes.

Last month, denominational officials quietly invited 200 of what Baptist press service later described simply as "wealthy Southern Baptists" for an upublicized visit to Washington. After a reception at the White House and a dinner at the Mayflower Hotel, which featured a 40-minute address by President Carter extolling missions, the group turned in their pledge cards.

The total promised to the mission program: $750,000.

The Southern Baptists' annual convention has little in common with the deliberative assemblies of most church bodies. "It's a combination of politics, camp meeting, placement service and family reunion," observed the Rev. William J. Cumbie of Alexandria, who heads the Mount Vernon Baptist Association and who served as parliamentarian at Atlanta.

Southern Baptist conventions do not have delegates, either from local churches or associations of churches, as do most denominational gatherings. The people who attend are called messengers.

Any Baptist church "in friendly cooperation with the convention's work" and that has contributed to the national body's budget, called the Cooperative Program, can send a messenger, according to denominational rules. Additional messangers up to the limit of 10 per church, may be sent, one each for every 250 members over a basic 250, or for each additional $250 contributed.

With a total of 35,255 churches, it is easy to see why there are only a few cities that can accommodate an annual convention. The meeting in Norfolk two years ago all but choked that city.

This year's convention, with a record registration approaching 23,000 persons, was, like most such gatherings, dominated by the clergy. Only 17 percent of the messengers were lay persons, and it is reasonable to assume that a fair number of them were wives of pastors.

For Southern Baptist pastors, the annual gathering is an opportunity to renew old friendships and make new ones. In the Baptist system, in which every local church has total authority to call or dismiss its pastor, such contacts and friendships provide the invisible network through which a minister learns of a vacant pulpit or other opportunity for advancement.

Resolutions, the be-all end-all of such denominations as United Church of Christ or Presbyterian Church meeting, also have their place in a Southern Baptist conventions. That place is minimal.

In its agenda for the three days of the convention here, the program committee had allotted 55 minutes to resolutions that might come before the convention.

But on the final day, debate on resolutions spilled over to absorb the entire lunch hour - at least for the few hundred faithful who sat through it.

Unlike the Presbyterians and the United Church of Christ, resolutions adopted by the Southern Baptists do not bind the denomination. Convention president Jimmy Allen of San Antonio reminded the body at one point that resolutions "just represent the folks who are sittin' in the room at that time."

Why, then, do Baptists adopt resolutions? "Resolutions are our understanding of the application of the gospel to life," was Cumbie's answer. It is also, he conceded wryly, a chance for "everybody to get his chance to say whatever crazy thing they want to say."

Allen, who presided with fairness, patience and humor throughout the convention, burst out laughing at one resolution, later resoundingly defeated by the convention. The measure proposed that Baptists demand an end to government deficit spending and pledge to vote only for public officials who would agree to resign at once if they ever voted to spend more money in a year than the government took in.

Allen quickly stifled his laughter, apologized to the many who had offered the resolution and explained: "I apologize, brother, I know this is a serious matter, but I just got to thinking about the rush of all those folks leaving Washington if your plan was ever put into effect."