ACROSS THE street from the White House lies Lafayette Park. Two worlds are joined and separated by Pennsylvania Avenue.

On the south side in a mansion at 1600 Pennsylvania Ave. lives Ronald Reagan. Across the street, without address or visible means of support, lives a man known as Thomas. He began his vigil here on June 3, 1981, and was there yesterday, three years later, his beard bushy, his hair held in a tail by an elastic band.

Mr. Reagan lives well housed AND well fed. The crumbs from his table would feed Thomas for a week. Thomas lives under the stars, exposed to rain and snow, summer and winter, without tent or sleeping bag. He feeds like the pigeons in the park, on what comes along. Sometimes he forages in a dumpster.

Mr. Reagan, across the street, issues moral messages against abortion and an evil empire, for freedom, the market and school prayer.

Thomas tends his messages night and day. They line the sidewalk facing the White House. signs OF all sizes and shapes, in various colors, with drawings of a mushroom cloud or skulls. Angled, side by side, they stand like a moral Maginot Line resisting the White House.

On Strike Till Bombs Are Dropped, Join Us, says one message. Another: Dear Uncle Sam, I Like You But I Hate Your Bombs. Let God's Children Go. Love, Thomas. And another: Mother Earth Loves You. Love Her Back Ban The Bomb. The messages go on, extending along the sidewalk by the edge of the park.

Yesterday I stopped to chat with Thomas, as I have done before. He is not a fanatic with burning eye and stabbing finger, a fevered Savonarola of Lafayette Park. No, he is soft spoken, intelligent, affable, natural, with gentle blue-grey eyes behind black horn-rimmed glasses, a man with the determination of an Ezekiel, the message of a Jonah, the disposition of a St. Frances of Assisi. Talking with him in the sunshine, I thought of the Hebrew prophets, the medieval monks - all people of uncompromising conviction, unqualified consistency.

Abandoned money

Now 37, he went to school to the eighth grade, but: has read Plato and discusses Socrates. He was a businessman, but day had a kind of vision. "Two ideas came to my mind: in order to be free, one must own nothing. And time is money." For him, time money meant that to save his time -- that is life -- he had to abandon money.

"Love of money is the root of all evil." He takes literally that biblical saying. He decided to live without money, went to one of the poorest parts of the world, walked, penniless, from Casablanca to Morocco to Cairo, not begging, eating what people gave him or what he found. He concluded that money -- evil -- Is not necessary for life.

One day, he concluded that states oppress people, Including his own country, the United States of America, so he threw his pass into the Thames in London and chose to a stateless person. But the world of states resisted. He spent a year in an Egyptian prison, was seized by the Israelis and expelled from the country. He was in jail in England and then deported to the United States. He protests that he is held In this country against his will.

He set up his vigil in front of the White House to communicate the wisdom he has learned. His occupation, now, is to be a philosopher. He speaks to the world by his signs, by conversations with passersby. He has gathered a little community of half a dozen people who. like him. are homeless for a cause, and live in Lafayette Park. He speaks, above all, by his example, his life of renunciation.

The police harass him and he has been arrested innumerable times, his signs confiscated are broken. He comes back, defends his right to free expression in the courts, makes more signs clings to his turf. He sleeps little, by snatches: if the police catch him in deep sleep they can charge him with camping. He insists he is not camping, but exercising his right "to speak out against the forms of political and technical insanity which presently threaten all life on Earth."