Posterity has a cruel way of over-ruling contemporary judgments of literature, but it is probably not too soon to say that the stories Alec McCowen told on the Eisenhower Theater's stage last night have enduring appeal.

He is an excellent storyteller, too-loud, clear and crafty. McCowen could turn the instruction manual for a clock radio into a fair evening's entertainment. So when he describes the beheading of John the Baptist-speaking King Herod's lines as a slobbering drunk and his stepdaughter's as a timelessly giddy, indifferent teen-ager-the power of the story is unmistakable.

But the evening that has filled halls and apparently transfixed audiences from Newcastle, England, to Cleveland, Ohio, hinges on something more than the story of jesus and McCowen's telling of it. Hanging in the air, as well, are 2,000 years of Christianity's transmission and expansion-from a few scattered, bickering colonies when John Mark set down his account in 65 A.D. to a force that today claims a billion followers, concentrated in the ruling nations of the industralized world.

Mark's Gospel, thought to be the earliest and the one drawn most directly from oral tradition, is short, artless and full of the primitive and mysterious force of Jesus' teachings.

What to make for instance, of the parable of the fig tree? Was it the tree's fault that Jesus and his disciples came along when the fruit was out of season? Did he cause the tree to wither and die from pique, or for the hypnotic effect of the deed on his followers, or simply to prove that, no matter how irrational the wish, unfettered faith could make it come true?

It is hard, suspended between the red plush floor and the red plush ceiling of the Eisenhower, to imagine yourself listening to an early Christian proselytizer telling these tales. But McCowen's accomplishment-reading just the lines themselves instead of between them-does somehow convey a sense of how these accounts, in this raw form, could have grabbed those first listeners.

After the one-man shows of Hal Holbrook and James Whitmore-full of whiskers and accents-McCowen's venture seems pretty daring in its simplicity.

He appears on stage looking like a man who has been to Brooks Brothers and left at least one salesman very contented. Then he tells a couple of jokes and, without warning or transition, starts reading his text-with no more dramatic gestures than an occasional hand thrust in the air, a wide-eyed stare or a giant step down-stage.

Some have criticized McWowen for polishing up the material a bit-tacking on a disputed appendix, for instance. And it is true that he plays to the fullest some humor that probably never was in the heads of John Mark or his Renaissance translators (as well as all the humor that was).

But anyone who objects to McCowen's occasional cutenesses will be grateful for his sustained, spectacular clarity. The ambiguities he leaves unsmoothed are, for the most part, those that intrigue as much as confuse a listener.

McCowen presents Jesus as a shrewd politician who avoided displeasing the authorities as long possible. He is low-keyed and businesslike, warning the Postles to keep certain assertions and explanations to themselves. But his voice gathers volume and melodramatic fervor when he describes how Jesus "rebuked the wind" and eyevitnesses asked, "What manner of man is this that even the earth and the sea obey him?"

At this, McCowenhs audience is almost as startled as the disciples must have been when they saw that stormy sea relax so suddenly.

The power of language lies, perversely, in its ability to obscure as well as to reveal detail. Reading in print the repeated episodes in which Jesus and the Apostles somehow managed to feed huge assemblies with a few loaves of bread and fish, one is tempted ot do a double take. Exactly where in the process of handing out all that food did it do its mutiplying?

The answer is simple. The story was made to be heard, not read. And a proper storyteller like the one now working at the Eisenhower - armed with yarns this astonishing - will keep his audience so mesmerized that no one will ever think to ask picky questions.

ST. MARK'S GOSPEL, a reading by Alec McCowen from the King James translation of the New Testament.

At the Eisenhower Theater through Sunday.