There is perhaps less than at first meets the eye in the new Vatican document disavowing Marxist and revolutionary brands of "liberation theology" in the Third World. But at a time when Americans are arguing over the place of religion in politics (or vice- versa), it is of keen interest.

"Liberation theology," a form of social activism with revolutionary overtones, has been the rage in recent years among certain rank-and-file Roman Catholic clergymen in Latin America and Africa.

The statement critiquing it is one more manifestation (others are doubtless yet to come) of Pope John Paul II's continuing campaign. He is sharply opposed to what must strike him as a truly bizarre heresy -- the contamination of Christian social teaching by a half- baked, often uncritically swallowed, Marxism.

As a Pole and a priest, the pope has spent much of his life resisting attempts of the communist regime in his native Warsaw to break the church to its bit. The pope thus knows his own mind more clearly than most on the subject of "revolutionary" theology, and on the more general relationship between sacred and secular.

He is, on the whole, a separationist. He has ordered American Jesuits such as Father Drinan out of Congress. In a memorable scene during his recent visit to Central America, he wagged a reproving finger at one of the sham priests who hold high positions in the Sandinista government of Nicaragua. (More recently, the Vatican has directed four such priests to leave office in Managua.)

No doubt some of the politician-priests of Managua are cynics. But guileless true believers, not cynics, are the threat. There are probably many more of them among the "liberation theology" clergymen in the Third World. They are typically priests who work among the poor and powerless, and who have accepted the idea that intractable poverty, rigid class structures and chronic misrule can be analyzed only through Marxist eyes and broken only by revolutionary alliances.

This gullibility among conscientious clergymen is easy to understand. They are daily witnesses to the changeless cycles of Third World poverty. But to those who believe that the "true" historical reality is class struggle, the new Vatican document offers two answers.

The theoretical answer is that "scientific" Marxism, with its stress on the class struggle and revolution, is fundamentally incompatible with Christian teaching, which sees the human heart as the seat of misery and misfortune. The practical answer is that while Marxism may have a specious appeal, it never delivers the "liberation" it preaches. Usually, it delivers the opposite: "Millions of our contemporaries yearn to recover those basic freedoms of which they were deprived by totalitarian and atheistic regimes . . . precisely in the name of liberation of the people. . . . Those who, perhaps inadvertently, make themselves accomplices of similar enslavements betray the poor they mean to help."

This struggle over "liberation theology" in the Roman church is obviously centered in those undeveloped societies where chronic poverty and misrule make apocalyptic solutions appealing.

But like the current debate over the pertinence of religion to political policy here in the United States, it asks the most basic and inescapable questions.

The issue of the proper relationship between sacred and secular never quite vanishes. Religious faith aspires to be relevant to its time, place and duties, yet without becoming so "relevant" (in the sense of merely faddish) or so confounded with secular doctrine, that it loses its force and genius. Pope John Paul II clearly sees a need to distance the church from political factionalism and fashion, though certainly not from its social mission, in Latin America and Africa.

Oddly, we seem to be headed in the opposite direction. There is a drive, led by President Reagan, to sanctify certain political views with the coloration of evangelical religion. So maybe the president would benefit from a look at the problems and pitfalls that have led the pope to try to disentangle legitimate Christian doctrine from the snares of secular politics.

At their worst, both the extremist "liberation theology" and the new politicized piosity in this country, are attempts to disguise human will and purpose with the alleged will and purpose of the Almighty. Now, as ever, it is tempting to dress Caesar in the armor of God.