When the gavel sounded the opening of the Republican National Convention, today one Virginia delegate was conspicuously absent from his seat.

Guy O. Farley Jr., the born-again Baptist who led the state's new right faction, spent the entire first convention session behind closed doors, helping plot a petition drive to persuade Ronald Reagan to make conservative Rep. Jack Kemp of New York his running mate.

For Farley, 47, a Warrenton lawyer, the maneuvering for Kemp was the latest in a series of controversial and highly visible moves that in one week have produced national headlines and, more important, placed him in a state leadership role from which he can seek his party's nomination for lieutenant governor next year.

Farely's sudden emergence began last week when, as one of Virginia's two representatives to the GOP's platform committee, he led the fight on two issues dear to the hearts of the party's conservative faithful: scrapping the Equal Rights Amendment and supporting an antiabortion constitutional amendment.

He helped steer the committee away from the party's 40-year stand favoring ERA, replacing a pro-ERA plank with a more general statement of support for women's rights that Farley helped write and that has mollified at least some of the party's female activists.

Farely was less gentle when it came to the abortion plank, leading a drive that threw out the vaguely worded antiabortion stand drawn up by Reagan strategists and replaced it with a strong statement of support for a "prolife" constitutional amendment.

Despite vehement opposition from party moderates, both of Farley's planks sailed relatively easily through the committee, a tribute, many said, to his skills as an organizer.

"I'd almost forgotten just how good Guy is," said Kenneth Kling, one of Reagan's southern states coordinators. "He knew how to talk to people and he knew how to talk to people and he knew how to use the rules and he was right in the middle of everything that went on."

But it is Farley's fundamentalist beliefs and connections, as much as his political skills, that have made him 3 key figure at this convention.Many Republicans see him as a crucial link between the party's traditional wing and the new right faction epitomized by right-wing Baptist preacher Jerry Falwell of Lynchburg.

"Guy is able to relate to both sides and that's very important," said Virginia Del. Lawrence Pratt, a Fairfax from going off the deep end."

Farley himself chafes at the notion that he is a political boss or a middleman for the fundamentalists.

"I have no ties to Dr. Falwell or any organization other than the Republican Party," Farley said today. "I do think it's good for the party to encourage new people to get involved. . . . [but] I don't think anyone should evr vote for anybody based on religious issues."

The Reagan people are less shy in discussing the Farley connection. John Alderson, Reagan's Virginia campaign coordinator, says that when the Reagan people decided in early 1979 that they needed someone as a bridge to the fundamentalist forces they thought could tip the nomination in Virginia, they went to Farley.

"Guy was the coordinator for what we call 'special groups'," said Alderson.

"He delivered, but the full potential was not realized because, as it turned out, we didn't need it."

A better test of the fundamentalists' growing power in the party, many Republicans believe, could come next year if Farley decides to run for lieutenant governor.

He is likely to face two opponents -- state Sen. Nathan Miller of Rockingham, a longtime Republican, and Tom Byrd of Winchester, heir to the Byrd family political throne -- who will tap traditional party and conservative support. For Farley to win, he will have to mobilize the born-again forces and press them into even greater action. h

"It would be a real test of strength," said Del. Warren E. Barry of Fairfax. "Next year, this is likely to be the only real battle Republicans will have."

The man the fundamentalists will be asked to support has undergone a metamorphosis since he first appeared on the political scene 20 years ago.

From 1962 until 1968, Farley represented Fairfax County in the Virginia General Assembly as a conservative Democrat. Although he ran as a law-and-order candidate, he was a popular defense lawyer with a reputation for getting clients out of tough spots.

He also had a reputation as one of the General Assembly's leading party-goers.

"He cut a wide swath through Richmond, or so I was told," said Barry, who entered the assembly the year after Farley left.

In 1969, Farley left the assembly to run for the Democratic nomination for attorney general. Although he was the choice of the party's conservative hierarchy, he lost in a primary to Andrew P. Miller, who went to win two terms.

Farley drifted away from the Democrats soon after that defeat -- "I didn't feel comfortable as a conservative in the party." By 1972, he was supporting Republican candidates.

After his political conversion, Farley underwent a religious conversion as well, becoming a born-again Christian in 1972. The two, he says, are totally unrelated.

"I made personal peace with God," he said. He prefers not to discuss his religious experience further than that.

Farley's new activism has not pleased everyone on the Virginia delegation. Some believe the strong antiabortion plank will alienate potential Reagan voters among Democrats. Others say his efforts for Kemp are putting unfair pressure on Reagan at a time when the former California governor should be left alone to make his own decision.

Farley disagrees, saying: "We need to show Gov. Reagan that the grassroots Republicans support Jack Kemp."