Pope John Paul II yesterday named as acting secretary of state Archbishop Agostino Casarioli, a veteran architect of the Roman Catholic church's detente with Communist governments under Pope Paul VI.

The appointment of Casaroli, 64, to the post that is second in power to the papacy itself is widely viewed as an indication that the Polish-born pope intends to pursue his predecessor's policy of agressive dialogue with Communist powers.

"It is an affirmation of the basic policy in Eastern Europe," one churchman here said of the appointment.

Casaroli, who will succeed French Cardinal Jean Villot, who died last month, has been secretary of the Council on Church Public Affairs, one of two major divisions within the secretariat and the one concerned with the Vatican's external relationships.

He has visited most of the Eastern European capitals "three or four times" and he is widely respected by the officially athleistic governments, according to one Vatican expert. He earned sharp criticism of militant anticommunist forces in the church for his key role in arranging the resignation of Hungary's Cardinal Josef Mindszenty in 1974 as well as for his representation of the Vatican at the 1975 Helsinki talks drafting accords on East-West relations.

Casaroli, who speaks four languages fluently and has a working knowledge of at least five others, handled most of the arrangements for Pope John Paul's visit to his native Poland next month.

Pope John Paul's selection of Casaroli is expected to be the first of a number of appointments both at the Vatican and at key posts around the world.

Several Vatican posts are held by men whose retirement can be considered overdue because of age, pressure of other duties or ill health. In addition, cardinals are lacking in nearly a dozen places around the world because of deaths or change of position.

Also announced yesterday was the appointment of Archbishop Giuseppe Caprio as president-designate of the administration of the Patrimony of the Holy See, an office concerned mainly with Vatican finances and church properties.

Caprio had been the subsecretary of state, a post characterized by one churchman as the "inside" arm of the state secretariat, with the public affiars office formerly headed by Casaroli as the "outside" arm.

Yesterday's appointments leave the two second-level posts in the state secretariat vacant, increasing Pope John Paul's opportunity to put his mark on the church during his papacy by naming his men to these key positions.

No new cardinals have been named for nearly two years. One unofficial Vatican source said yesterday that the announcement of John Paul's first cardinal appointments would be made "in two or three days."

Practically, the only function of a cardinal, apart from his role as bishop or archbishop, is to elect a pope. Nevertheless, the office carries with it enormous prestige. Church regulations require that key Vatican posts, such as secretary of state, must be filled by a cardinal.

It is for this reason that Casaroli was named "acting" secretary of state until he is made a cardinal.

The church has been represented by one or more cardinals in every part of the world where there are substantial numbers of catholics. Currently, there are 11 locations lacking cardinals that have previously had them: Wellington, New Zeland; Krakow, Poland; Turin and Venice in Italy; Armagh, Ireland; Osaka, Japan; Baltimore, Mexico City, Bombay, Hanoir and China.

It is unlikely that the Vatican would name cardinals in the last two places, given the political situations there.

There are four posts at the Curia, the Vatican's adminstratice body, now headed by cardinals who are over the retirement age of 75, overworked, or in ill health.

The Sacred Congregation for Catholic Education is headed by Cardinal Gabriel-Marie Garrone, 77.

The Sacred Congregation for Causes of Saints is headed by Cardinal Corrado Bafile, who will be 76 in July.

Cardinal John Wright of the United States, who heads the Congregation for the Clergy, is only 70 but suffers a severe nervous disorder that threatens to immobilize him and he is expected to resign before reaching 75.

The Secretariat for Christian Unity is headed by Cardinal Johnannes Willebrands who, three years ago, was also named primate of Holland. It is a post that would be considered full time even without the added problems of prolonged battling between liberal and conservative factions in the Dutch church.

Yesterday's papal appointments were viewed in the light of at least two studies initiated by Pope Paul, and recommendations for the restructuring and streamlining of the Vatican's cumbersome and top-heavy administrative machinery.

The state secretariat, with responsibility for day-to-day management of the Vatican as well as foreign affairs, had become particularly cumbersome under Villot and his powerful deputy, now Cardinal Archbishop Giovanni Benelli of Florence. It was Caprio, now moved outside the secretariat, Who succeeded Benelli. CAPTION: Picture, AGOSTINO CASAROLI. . . architect of eastern policy