Christian churches should advocate courageous moral leadership in society whatever the costs, a leading South African minister and prominent opponent of apartheid told an audience of 70 in Memorial Church yesterday.

In a 15-minute sermon, the Rev. Christiaan Frederick Beyers Naude said that although he was brought up to believe in apartheid, he gradually decided that the system conflicted fundamentally with Christian values.

"I preached apartheid, I believed apartheid, I practiced apartheid, and I enjoyed apartheid," Naude said, "but I came to understand that it was utterly incompatible with my ideals as a Christian."

At the time, Naude held a leadership position in the Dutch Reformed Church, but in 1964 he founded the Christian Institute, an ecumenical body meant to use religious organizations to combat apartheid. Naude formally left the Dutch Reformed Church in 1980.

Alythough Naude said his work has met with severe harassment from police and government officials, he said his faith in his organization's mission remains intact.

"The present aim of the Christian Institute is reconciliation, between Black and white, between Black and Black, and between white and white," Naude said. "Forgiveness is the key to this reconciliation, and it is the duty of South Africa's churches to encourage a positive spirit among all sides."

Naude was introduced by S. Allen Counter, director of the Harvard Foundation, which sponsored the speech in conjunction with the Robert F. Kennedy Foundation. Dr. Counter praised Naude's work at the Institute, noting that Oliver Tambo of the African National Congress had once referred to the minister as "a friend to all of South Africa, but an especially fine friend to South African Blacks."