MOST AMERICANS now living had not been born when William Brennan was appointed to the Supreme Court by President Eisenhower in 1956. For 34 years, during which constitutional rights and the powers of government were continually tested, Justice Brennan has played a leading role as a defender of individual liberty and civil rights. His resignation from the court on Friday is a blow to those who admire the man and share his core view of a citizen's rights in a constitutional democracy.

Longevity alone does not make a great jurist. Justice Brennan did far more than endure. From his first days on the court, he set a course as a protector of the unpopular and a champion of those political, religious and racial minorities whose rights are most likely to be challenged by a powerful majority. His opinions were clear and unequivocal, and he was a leader, whether writing for the majority or strenuously objecting in dissent. In cases involving alleged subversives, underrepresented voters, racial groups seeking justice and opportunity or the poor and handicapped suffering discrimination, he saw the Constitution as a shield against oppression. The rights of criminal defendants were greatly expanded during his years on the court, in part because of his insistent concern that constitutional standards be observed. His unswerving opposition to the death penalty exemplifies his conviction that the state owes an obligation even to the most despised felons.

Justice Brennan's staunch defense of the First Amendment is legendary. He has understood that a free press is a right that belongs not to publishers or reporters, but to the people. He has been vigilant on behalf of religious minorities and firm about the separate roles of church and state. Freedom of speech for dissenters, from fascists and communists to flag-burners, has been of paramount importance to him, and he brooked no censorship of political debate.

We have had occasional differences with Justice Brennan. But none has changed our perception that he has an ability to see litigants as individuals and to view the courts as an instrument of justice that has graced his whole career.