''We're present at the making of history,'' Walter Cronkite told the crowd almost filling the vast Cathedral Church of St. John the Divine here. But this time Mr. Cronkite was anchoring, so to speak, an extraordinary literary occasion rather than a flight into space. It was Monday night's service of dedication for an American Poets' Corner to honor American writers, as the Poets' Corner in Westminster Abbey has been honoring British writers ever since Geoffrey Chaucer's tomb was erected there in 1556, more than 150 years after his passing.
The American Poets' Corner has not waited quite that long to honor Emily Dickinson (1830-86) and Walt Whitman (1819-92). These are the first two poets chosen by a committee of electors charged with voting for two writers each year to be memorialized in the cathedral's Arts Bay. This year, Washington Irving ( 1783-1859) made a third; he had been previously selected in connection with his bicentennial.
If Irving's Rip van Winkle had waked up during the ceremonies the other evening, he might have really rubbed his eyes, not to mention his ears.
For there, targeted by TV lenses, with the tearing sound of technicians' Velcro occasionally breaking the hush, was the commanding figure of Gregory Peck , lending his actor's eloquence to Irving's tale of Ichabod Crane, Brom Bones, and the headless horseman.
And, in addition to sacred music played by Rosalyn Tureck and others (with the New York Philharmonic's Zubin Mehta as honorary conductor), there was the soaring soprano saxophone with which Paul Winter so often accompanies the dulcet voices of whales and wolves. Now, as the procession started toward the unveiling of the Poets' Corner, Mr. Winter stood in the dark upper reaches of the cathedral, almost like a figure on a cornice, the bell of his horn making a golden circle in the shadows. Then he played a kind of celestial jam session with the cathedral's organist, Paul Halley, their melodic lines intertwining until the organ reached a triumphant crescendo as the procession arrived.
''O Rare Emily Dickinson!'' proclaims the carving in the floor that was soon under the feet of the thronging poets and other people. Whitman's stone bears his line, ''I stop somewhere waiting for you.'' Irving's stone describes him both in literary terms and as ''beloved squire of Sunnyside.''
On the wall above, in 12 feet of graceful script by master stonecutter John Benson, are the Psalmist's words, ''My heart is inditing a good matter; my tongue is the pen of a ready writer.''
For all the evening's admiration of the enduring monument now begun - and President Reagan joined in the messages of endorsement - everyone involved obviously knew where writers truly endure: in people's minds and hearts. Writers make our existence ''understandable to ourselves and to our neighbors,'' said the Very Rev. James Parks Morton, dean of the Cathedral Church of St. John the Divine. The evening's honorary artist, choreographer Martha Graham, sent word that poetry, to her, reveals the ''inner landscape.'' And when poet Robert Penn Warren read from Whitman's tribute to Lincoln, ''When Lilacs Last in the Dooryard Bloom'd,'' it was clear once more that the poem was about more than a bush ''with every leaf a miracle.''