The 600 Christian fundamentalist parents who sued Alabama school officials, contending that public school textbooks promoted the "religion" of secular humanism, will not appeal their case to the Supreme Court, their attorneys announced yesterday.

The attorneys said that "uncertainty over the composition" of the court -- who will fill the seat vacated when Justice Lewis F. Powell Jr. retired last June -- prompted the decision. President Reagan's third nominee for the post is U.S. Circuit Court Judge Anthony M. Kennedy.

Attorneys also said it was unlikely that the Supreme Court would accept the case for a hearing. Attorney Bob Sherling said in a written statement that the failure of the appeals court to review testimony and exhibits from the district court trial would require the Supreme Court to make a lengthy review.

"This makes it unlikely that the Supreme Court would accept this case for hearing since the court prefers to limit its function to just reviewing conclusions of law," he said.

In March, U.S. District Court Judge W. Brevard Hand banned 44 textbooks from use in the classroom, asserting that they violated the First Amendment prohibition on government establishment of a religion. Hand's ruling was overturned in August by a three-judge panel of the 11th U.S. Circuit Court of Appeals.

At the time, the parents said they would appeal to the Supreme Court.

"We are pleased with the results of this litigation . . . . The trial court found that secular humanism is a religion; the Court of Appeals did not reverse this finding," Sherling said yesterday.

The appeals court said it was not necessary to address the question of whether secular humanism is a religion, ruling that the parents had not adequately shown that the schools were promoting a religion in violation of the Constitution.

Attorneys for the Alabama Board of Education and parents who intervened on the board's behalf said the decision not to appeal vindicated their arguments.

"The board's position has always been that they didn't violate anyone's constitutional rights," said James Ippolito, attorney for the state board.

The appeals court, based in Atlanta, ruled that Hand had erred in his opinion and instructed him to dissolve his order banning the books and dismiss the lawsuit. That decision allowed the state's schools to use the books this school year.

The parents had argued that 39 history and social studies texts and five home economics books promoted secular humanism, which the dictionary defines as a human-centered movement based on the premise that it is possible for people to live ethically without a supernatural being.