The Rev. R. L. Maddox is fixing to leave the Calhoun, Ga., First Baptist Church, where he has preached to the likes of the town banker, Bert Lance, and a local lawyer, Jack Carter, to become a speechwriter for Jack's father, the president of the United States.

"I do it out of a sense of call and responsibility," said Maddox says. The call came from Gerald Rafshoon, who is Jimmy Carter's director of communications.

Maddox says that he has "done a good bit of writing" and presidential aides say Carter needs a bit of good writing, so there is a real chance that this geographically higher calling will work out for all concerned.

"I have great respect for the president," Maddox said in a telephone interview."I feel this opportunity is an extension of my call as a minister. My understanding of the will of God embraces this sort of thing for me."

The First Baptist Church of Calhoun is a sizable institution, having 850 members, some of national renown Jack and Judy Carter are members and friends of his, the preacher said. Bert Lance lives right around the corner, and although he is not a member of the church, Maddox says he has worshipped there from time to time.

Maddox says it was just two weeks ago-"April 2nd, I remember the date well"-that Rafshoon asked him to the White House. "Mr. Rafshoon called and invited me to come up to talk about what he called 'a matter of mutual interest.' That was all he said about it, but I had an idea of what he had in mind."

He had an idea because he had done some writing for the Carters before.

Back last June, Jody Powell asked him to help write the speech the president gave in Atlanta to the Brotherhood Commission of the Southern Baptist Convention. Then he wrote a speech on "the family" for Rosalynn Carter to deliver in Orlando before the Christian Life Commission of the southern Baptist Convention. And more recently, Maddox said he wrote a draft for Carter's speech on the White House lawn at the signing of the Egyptian-Israeli peace treaty. The president "took a lot of the material I sent him" and used it in his speech, Maddox said.

Maddox knew Powell from his days as Baptist preacher in the press secretary's hometown of Vienna, Ga. In the mid-'60s, Maddox said, Carter preached from the pulpit of his church in Vienna.

Maddox is a graduate of Baylor University and received a doctorate in sacred theology from Emory University in Atlanta.

Rafshoon confirmed yesterday that Maddox will be joining his staff of White House speechwriters. And Maddox confirmed that he will be moving his wife and three children to Washington "just as soon as I can find us a place to live."

He is leaving a calling that pays $20,000 a year-but provides him with a home-for one that pays $37,000 but leaves him to fend for himself in the high-rent capital of the nation. He has already heard all about the housing prices in Washington and suburbia: "To everybody I talk with, I say, 'Hey, help me!"

And of his new job, he adds:

"I'm scared. But I'm comfortable with this. The will of God unfolds in this way for me."