BRENDAN By Frederick Buechner Atheneum. 240 pp. $17.95

By the 6th century A.D. civilization was dead in Europe. The ancient Mediterranean culture was exhausted and incapable of generating the new ideas and techniques necessary to lift men above the condition of beasts. The cities were rotting and empty; much of the countryside was wasteland; even Christianity, last and greatest of the intellectual inventions of the antique, was disintegrating into a chaos of heresies and persecutions.

Still, a few remnants of philosophy survived, and these seeds would grow, through a few generations, into the thriving and inventive culture of the Middle Ages. One of these seedbeds was the holy land of Ireland, where would spring up a lusty crop of vigorous minds, committed to reason and order, whose influence would revive learning throughout Europe.

Frederick Buechner's novel "Brendan" works the ground at the beginning of this Irish blossoming. Its hero is the wonderful 6th-century saint called the Navigator, most famous for his journeys in quest of Tir-na-n-Og, the fabled Land of the Ever Young, where neither death nor age had ever come, and which lay far to the west, beyond the Atlantic, in a land called Hy Brasail.

To control this exuberant material, Buechner uses a first-person narrator, Finn, who is nonetheless invisible to us: We never learn much about Finn, save that he loves Brendan enough to follow him anywhere. Finn's viewpoint is naive and literal -- perhaps too naive, too literal; it is distracting to have an educated writer produce sentences like "She drew her knees up tight to her and laid there squealing like it {a spear} had gone clear through her."

Nonetheless I concede the strategic value. Finn accepts, and tells, the fabulous tales of Brendan on the same level as he does ordinary and everyday matters, which is not to say Buechner makes the mythic ordinary so much as he makes the ordinary very strange indeed. Moreover, in this the story finds its driving force -- the extremes of its realities, the contrast between the uplifting and transforming energy of Brendan's Christian vision and the harsh and hopeless lives of the pagan Irish, whose indifferent gods notice them only to beat them into the dirt for failing their ritual duties, and to whom Christ comes as the Light of the World.

Thus, not as metaphor but as utter reality, we get Brendan's first voyage, when, casting away his steerboard, he lets the wind of Heaven take him within sight of Hell:

"He was full of smoke with clouds on his summit. He was shooting flames higher than himself. They were falling back down on him sending such a spray of sparks and embers the air all around was ablaze and I feared for our sails. One of his flanks was cracked open. There were gold wide streams of fire seeping out of it. They slipped slow as mud over the crags and made a fierce seasnake hiss when they reached the water. Clouds of steam rose and went steaming over the surface. All the while there was the sound of monstrous bellows and a great booming and thumping like a thousand smiths."

Properly enough, Brendan never finds Tir-na-n-Og. What he does find beyond the sea drives him past his young man's longing to escape from the horror and despair of the world, to the peace and truth at the core of Christianity -- the commitment of people to helping other people, the unique value of every soul, expressed not in self-glorification, but in recognition of its aloneness and helplessness against a world of fury and death.

There are flaws in this novel, beyond Finn's grammatical errata. Buechner transports Brendan to England, to witness the despair and degradation of a King Arthur too sketchy and too ordinary to serve much fictional purpose. I felt no change or growth in the narrator Finn that would give me another grip on Brendan. But the saint himself -- and his fellow saints, because Ireland in these days seems to have been crowded to the gunwales with saints -- come through as human volcanoes, erupting with the newfound fire and excitement of a living Christ. In our own time, when religion is debased, an electronic game show, an insult to the thirsty soul, Buechner's novel proves again the power of faith, to lift us up, to hold us straight, to send us on again. The reviewer's most recent historical novel is "The Belt of Gold."