"How many years will I need to digest Mexico? How many lifetimes must I have lived to comprehend it? But one consolation is left and it suffices me. I need not more than a minute to love it." -- Erico Verissimo

I RETURN TO los Estados Unidos Mexicanos -- land of Moctezuma -- from time to time on assignment, and it is always an adventure. For Mexico, if approached with a receptive mind, turns travel into an experience in living.

Our Latin neighbor, a centuries-old mix of pre-Columbian Indian root, Spanish rule and Mexican uniqueness, is a kaleidoscopic, contrast-filled country. It can tantalize with its mystery, excite with flamboyant variety, surprise with vitality, dismay with poverty, educate with history and culture, and repay friendliness with warmth. Happily -- since their nation and ours share a common border -- we gringos are especially welcome.

But which Mexico is your taco? There are many Mexicos: Mexico City (originally Tenochtitlan), Cuernavaca, Taxco, Acapulco, Puebla, Guanajuato, Guadalajara, Oaxaca, Merida, Cancun . . . Teotihuacan, Monte Alban, Chicen-Itza . . . From sophisticated, problem-prone capital to preserved colonial area to sunny beach resort to impressive ruins and artifacts of ancient civilizations such as the Toltec, Aztec, Zapotec, Maya.

The language is Spanish, richly expressive, melodious and (some say) more beautifully spoken than anywhere else in Latin America, or even in Spain. If you know only "gracias" and "adios," though, you need fear. English is generally understood on the tourist trail, but the polite Mexicans will not take offense or make snide remarks if you mangle their language.

At the Universidad Nacional Autonoma de Mexico, a grandmotherly profesora once patiently admonished me:

"Remember, 'El Papa' is the pope, 'el papa' is the father, and 'la papa' is the potato. Please don't mix them up!"

Mexican music, from mariachi brass to gentle strings, speaks a language all Americans can understand. Some of our loveliest ballards, "standards" that have given us pleasure for decades and put to shame the short-lived inanities that now often pass for pop songs, were actually written by Mexican composers and later given English lyrics.

Jose de la Cruz Porfirio Diaz Mori, that crusty, lavishly mustachioed general who ruled Mexico as a dictator for more than 30 years -- until the Revolution of 1910 forced him into retirement at 80 -- is credited with coining the cynical phrase "Pobre Mexico, tan lejos je Dios y tan cerca de Los Estados Unidos" (Poor Mexico, so far from God and so close to the Unites States). It is true, of course, that Mexico still remembers bitter incidents of internal interference by this country in the distant past. Indeed, the new 5,000-peso bill (about $220 U.S.) depicts Chapultepec Castle in Mexico City and the six teen-age military students who died there while fighting U.S. troops during the 1847 invasion of Mexico. (One youth wrapped himself in the Mexican flag and defiantly threw himself off a parapet rather than surrender. The Day of the Heroic Children, Sept. 13, is a Mexican holiday.)

Most fortunately -- for both countries -- such proud awareness of a troubled history has not been allowed to become dangerous recrimination in the present.

Don Porfirio, if alive today, would be amazed to discover that Mexican tourism, "la industria sin chimineas" (the industry without smokestacks) last year drew a record 4.2 million visitors, of which approximately 3 million were Americans. And while only 2.6 million Mexicans visited the United States in 1980, they greatly helped our balance of payments because they spent the same amount here as we spent there: $2.5 billion.

For the more financially oriented and perhaps less romantic traveler, there are still good dollars-and-cents reasons for vacationing in Mexico at this time if your food and lodging demands are reasonable. Though inflation has hurt the average Mexican far worse than the American, and prices for accommodations and food have naturally risen sharply (a fact particularly noticeable at luxury hotels and restaurants), you currently get about 23 pesos for each dollar. Also, the U.S. Congress recently removed restrictions on income tax deductions for conventions held in Mexico. This may help raise the total number of American visitors after the 10 percent decline last year from 1979.

But if your primary reason for heading south of the border is to save a buck, you will be shortchanging yourself if you are unaware of all that Mexico offers. As the popular saying goes: "Como Mexico, no hay dos" (There is only one Mexico).