The Southern Baptist Convention turned thumbs down on the Equal Rights Amendment in a resolution adopted here yesterday.

Messengers, as delegates to the convention are called, swept aside the bland proposal of their resolutions committee, which urged persons to act "in light of their best informed judgement." They adopted instead a substitute resolution denouncing as "illegal" current efforts to extend the March 22, 1979, deadline for ratification of the ERA.

"ERA is a big power grab by the federal government," declared Sue Sum of Atlanta. She was in the midst of expounding her conviction that the proposed constitutional amendment would give the government control over "marriage property rights, divorce and child custody" when she was cut off by convention President Jimmy Allen because she had exceeded the one-minute limit the convention imposed on discussion. "But you got a lot said for a Georgia girl in one minute," said Allen.

An even more controversial question among Baptists - abortion - was resolved after extensive debate, by a resolution reaffirming positions adopted by earlier conventions. The resolution affirmed the "scaredness and dignity of human life," rejecting abortion for "selfish and nontherapeutic reasons," but stopped short of condemning all abortions.

Another resolution called on President Carter to "veto any (private school tuition) tax credit legislation now under consideration by Congess," and directed Baptist agencies to lobby against such legislation as a "threat to the First Amendment guarantees of non-establishment of religion and the free exercise of religion."

Still another resolution formally expressed Baptist concern over an Israeli law adopted last year, which makes it a crime to offer material inducements to persuade a person to change his religion.

Despite the range in magnitude of the topics covered, discussion on the resolutions occupied only a small fraction of the time of the conversation which concluded tonight. The overwhelming focus of the program, in sermons, songs, and message-laden pageants, was on the church's theme "Bold Mission Thurst," a campaign to spread the gospel to the entire world by the year 2,000.

In an address to the convention this afternoon, Coretta Scott King told the overwhelmingly white church gathering "that in the rush to bring the spiritual message of Christ to our brothers and sisters around the world, we must not forget that we have to care about their physical and mental peace as well.

King, who was introduced by Atlanta Mayor Maynard Jackson, was substituting for her father-in-law, Martin Luther King Sr., who is touring and preaching in Hungary. In her address, she dwelled at some length on the failure of white southern churches to support the efforts of her late husband and other civil rights leaders to combat racial discrimination.

She urged Baptists today to be as concerned about social needs as about saving souls. "We cannot, I believe, talk about a spiritual awakening . . . without understanding that we must awaken our old consciences to the needs of our brothers and sisters, to their physical as well as their spiritual needs," she said.