ONE IS tempted to call it a miracle. Franco Zeffirelli's "Jesus of Nazareth," a six-hour dramatization of New Testament events, is not only one of the most produced for television, it also represents a substantial improvement over every other large-scale movie project based on the same incomparable and unwieldy material.

The firm will be shown in two installments of approximately three hours each on Channel 4 and other NBC stations tonight and next Sunday at 8 o'clock. The first half will actually run until about 11:12 p.m. NBC has promised not to cut any of the film.

As director, Zeffirelli occasionally goes overboard on the reverence, the film almost freezing into a fresco and gasping for air. His exemplary visual instincts, however, tend to redeem even the more sluggish moments; he has an eye for composition and lighting that is unprecedented in television films, even in such acclaimed productions as "Roots" and "The Autobiography of Miss Jane Pittman."

As screenwriter, in collaboration with Anthony Burgess ("A Clockwork Orange") and Suso Cecchi d'Amico, Zeffirelli manages a brilliant balance between biography of Jesus and the political and social context in which the story takes place. There may never have been a religious film with so pervasive a sense of place and period.

At the same time the intrinsic dramatic and symbolic values of the story, which have eluded past filmmakers, come through with striking clarity so that, as with "Roots" earlier this season, it is likely that the television audience will find itself moved by an intimate experience to a degree theatrical movies seem hard put to equal.

Zeffirelli has told the tale with revolutionary taste and austerity. There are no lightning bolts, marching Roman legions, earthquakes or floods. There are no special effects. What Zeffirelli sacrifices in show-bizzy pizzazz he makes up for in human impact. There were justifiable groans when the project was announced in 1975 because, except perhaps for Pier Paolo Passolini's low-budget "Gospel According to St. Matthew," this biblical material has usually defied subtle film treatment, and there would be no point to another live-action cartoon or Classics Comic approach. Television, in addition, does not lend itself to gigantic spectacles in which hordes of ant-sized people sweep across silly vistas.

What has come to pass justifies taking another whack at an oft-told story. And after years of such pop religious phenomena as the stage and screen versions of "Godspell" and "Jesus Christ Superstar," in an era when electronic prophets use TV talk shows as pulpits, in a decade that frequently presents religion as just another leisure-time industry, the straightforward, simple, fundamental approach taken by Zeffirelli seems refreshingly ungimmicked and unapologetic.

Among the aspects of the life of Jesus that are intelligently covered is his status as a media sensation in a time when the major mass media were the shout and the rumor. Jesus was controversial - and his capacity for offending the establishment is convincingly portrayed.

Of course, it's no small irony that this version of the life of Jesus proved controversial itself with some present day funadmentalist religious groups, who took the not-unheard-of step of condemning it before they had seen it.

The film was denounced on the basis of a published interview with Zeffirelli and a review of the picture in, of all sources, Modern Screen magazine. Though other religious leaders subsequently rallied around the film, the hint of controversy, coming soon after the taking of hostages in Washington by a religious sect protesting, among other things, a film about its leader, scared off General Motors, which was to be the sponsor of the two-part telecast.

GM had already sunk $3 million in production funds into the $12-million project, so its decision was not only bad behavior, it was bad business. But Procter and Gamble, one of the largest buyers of TV time, picked up GM's option on the show and did it at a reportedly "bargain" rate.

Early this week, officials at Procter and Gamble's Cincinnati headquarters still had not decided where they would place commercials within the program. Since the company's 50 different consumer products include such items as Charmin bathroom tissue, it was conceivable that the Sermon on the Mount could be interrupted with a squeezing seminar by the ubiquitous "Mr. Whipple."

But a company spokesman said, "We're looking at the whole subject with a great deal of care," and suggested some degree of sensitivity would be evident.

In a classic programming maneuver, meanwhile, ABC announced that is had scheduled a rerun of the 1956 Cecil B. DeMille biblical blockbuster "The Ten Commandments" for 8 p.m. on Easter Sunday, directly opposite the second half of "Jesus of Nazareth." Zeffirelli's film, as it happens, is a direct refutation of the fire and brimstone approach to biblical material popularized by DeMille, who filmed his own bombastic life of Jesus, "King of Kings," in 1926. This became the standard movie version, shown for years afterward in churches as well as movie theaters and on television.

In its immensity and maybe even in its vulgarity, "Commandments" and similar Hollywood religion films could be moving, but not in the immediate way that Zeffirelli's film is. One doesn't sense cynical showmanship bhind it, and its excesses seem excesses of fervor or enthusiasm, not of calculation for effect. Though its cinematography of Morocco and Tunisia is breathtaking, it's clear that Zeffirelli is more concerned with a vast and convincing landscape of fascinating human faces.

"Nazareth" even survives, for the most part, the fact that it is a Lew Grade all-star production. One is never quite so jarred as when John Wayne popped up as a centurion in George Stevens' "The Greatest Story Ever Told," although things do get to be a bit much in "Nazareth" when Anne Bancroft makes her appearance as Mary Magdalene.

That's the only bit of casting that really seems reckless. Otherwise, Zeffirelli not only makes the most of brief appearances b big names, he manages to make such lesser-magnitude performers as James Farentino (Simon Peter) and Ernest Borgnine (Roman soldier) credible and fresh.

The stellar names include Laurence Olivier as Nicodemus, Donald Pleasmer as Herod Antipas. Anthony Quinn as Caiaphas, Ralph Richardson as Simeon, Rod Steiger as Pontius Pilate, Peter Ustinov as Herod. Michael York as John the Baptist, James Mason as Joseph of Arimathea, James Earl Jones as Balthazar and Cyril Cusack as Yuhuda - most of them resplendent in photogenic beards.

As the Virgin Mary, a role nothing if not difficult, Olivia Hussey is simultaneously fragile, mystified and strong, and Zeffirelli's first shot of her, as she works behind a loom, is mesmerizing and touching.

There are serious problems, however, with the protrayal of Jesus the man by Robert Powell, an actor whose film work includes the title role in the little-seen Ken Russel romp, "Mahler." Powell's appearance is a drawback here. Though it may be commendable that he is no way resembles the fair-haired Aryan stereotype dominant in such films for years, Powell looks so wan and gaunt that one fears for his health. His eyes seem hollow.

Powell's speaking voice is practically perfect, and one can easily accept that he could spellbind an audience, but under Zeffirelli's direction he speaks so slowly and arduously that he become something of a drag. He is not as commanding a presence as one might expect, but for actors, this role has always been a trial, especially when one considers that any attempt at characterization might be taken by some spectators as blasphemy. Perhaps Powell has done as well as possible under such circumstances.

This movie Jesus performs miracles, but low-profile miracles. He does not walk on water. He does not turn water into wine, though he does turn a few fishes and loaves of bread into adequate rations for a throng. In one extremely moving and well-shot sequence, he revives a young woman thought to be dead with "Rise, little girl," and later raises Lazarus from the tomb. It is clearly the firm's point of view, contrary to what its premature critics may have said, that Christ's claims to divinity are true.

Zeffirelli and his fellow writers don't strain for modern parallels and "relevance" here, but let them arise naturally from the story. Ustinov's Herod might be any modern-day political boss. Steiger's Pilate is a man struggling with the expediencies of juggling liberty and order, with an emphasis of course on the latter. Ian McShane's Judas is not a dirty little traitor but disillusioned in his attempts to manipulate Jesus as a political figure and duped into a betrayal that he thought to be an act of practical public relations.

Occasionally the writers imbue characters with an unlikely prescience and have them voice unlikely ruminations.It doesn't ring very true when Matthew remarks to Simon, "We'll never be the same," after encountering Jesus, "and neither will the lives of everyone in the whole world." That's not only unlikely; it's clunky.

Zeffirelli went off the deep end of dreamy romanticism with his film "Brother Sun, Sister Moon," in which St. Francis of Assisi and his followers were depicted as the forerunners of '60s, political activists. They did everything but makes the peace sign. This indulgence has been tempered in "Nazareth"; the themes of youthful radicalism and challenges to authority are dealt with reasonably, and Michael York's portrayal of John the Baptist as an obsessed young firebrand is one of the most effective performances in the film. Since Zeffirelli wanted to avoid having a booming Voice of God intruding from the heavens, he gives John one of God's lines: "This is my beloved son in whom I am well pleased." Some clergymen have objected to such liberties, but they seem minor in relation to the whole work.

And the work is, despite flaws, a truly impressive accomplishment, all the more so for the pitfalls it avoids, and all the more affecting for an aura of spirituality that seems entirely heartfelt. Zeffirelli has produced an emotional and resonant epic that does more than tell a very good story very well. "Jesus of Nazareth" is about the timeless human dreams of life beyond death and refuge from pain; the real beauty of it is the real beauty of it.