Mother M. Sixtina of Alton, Ill., apologized to Pope John Paul Ill in The Washington Post today for what she called "an insult" by another nun during his Washington visit.

Mother Sixtina said today she was in a pew at the National Shrine of the Immaculate Conception when Sister Theresa Kane, a member of the Sisters of Mercy, urged the pope to reconsider his ban on ordination of women.

"I was very embarrassed and I saw the same reaction by the other nuns around me. When I got home, the [religious] community said, "Why didn't you do anything about it? It was an insult to the Holy Father.'"

So Mother Sixtina, provincial superior of the Sisters of St. Francis of the Martyr St. George in Alton, placed an 10 1/2-by-6-inch advertisement in today's editions of The Post, "We, who most likely speak for the large majority of religious women in the United States, apologize to His Holiness John Paul II for the public rudeness shown him" in Washington, the ad said.

It charged that Sister Theresa was "not only impertinent [to the pope] . . . but she has also offended the millions of us who love him and gladly accept his teaching."

The ad, which cost $2,940, was paid for by the order, which runs the 203-bed St. Anthony's Hospital in Alton, and by the Immaculate Heart of Mary order of teaching nuns of Philadelphia, which Mother Sixtina said contributed at her request.

"We wanted the lay people to know that we agree with His Holiness," Mother Sixtina said. "Since the whole thing was spread so largely across the country in the papers, we thought the answer should go across the country also."

Mother Sixtina said the ad was her idea and that her superiors at the 2,000-member order's world headquarters in Thuine, West Germany, did not know about it yet. "But I know they will support me," the Berlin native said in a soft German accent.

She added she had discussed contents of the message with other administrators at the Alton convent, and then informed the rest of the nuns. There was no disagreement, she said.

The order also teaches children of the hospital staff. The hospital, one of three in the town of about 45,000 an hour's drive northeast of here, functioned as a nursing home when the nuns founded it 50 years ago; it specializes in pediatric medicine now.