ON A June day in 1982, outside a cathedral in the black township of Soweto, a huge white policeman beating an elderly black man with a stick suddenly found himself confronted by a small black man in clerical robes who held a cross aloft until the beating stopped. It was Bishop Desmond Tutu of South Africa performing, at high risk, the mission of mercy and justice for which he yesterday received the Nobel Peace Prize. Never has the honor gone to a worthier person than the 53-year-old Anglican bishop and general secretary of the South African Council of Churches -- perhaps the most widely accepted black critic of apartheid permitted to operate in his country today.

Two qualities distinguish Bishop Tutu. The first is the purity and force of the moral passion he directs at South Africa's system of legalized racism. The second is his deep commitment to nonviolence. So outrageous and total is apartheid's assault on human dignity that it is always a marvel to Westerners who look in on South Africa that there are any blacks left with the magnanimity and patience to support peaceful change. Bishop Tutu is among them. He has a vision of a society in which individuals, equal already in God's eyes, become equal as well in the eyes of the law. He accepts "that things may come to such a pass that people feel compelled to resist them violently. My purpose is to try to keep that to a minimum."

From Johannesburg yesterday, there was no official reaction to the Nobel award. While millions of South African blacks will probably find in it encouragement for the cause of racial equality, many whites, especially Afrikaners, will see it as one more stroke of intervention by a hostile outside world.

It is too bad that their inability to understand is so rock hard and so deep. For surely this is a moment when a wise South African leader would respond in another way. President P. W. Botha, of course, portrays himself as a reformer, though his new constitution utterly ignores blacks and drew only minimal Asian and Colored support. At his inaugural last month, for instance, he held open, in his maddeningly hedged fashion, the possibility at least of political consultations with blacks. Bishop Tutu is a fit interlocutor -- proven, everywhere respected and justly so.

President Botha can easily denounce the bishop and his Nobel prize. How much better it would be for all South Africans, not least whites, if he could bring himself to speak of the bishop and his prize in a way that indicated some understanding of the moral fervor of the man and of the torment that the policy of apartheid brings not only to South African blacks but to people everywhere. That torment and Desmond Tutu's fervor to end it are what the award of the Nobel prize is all about.