Pope John Paul II has reassigned U.S. Apostolic Delegate Jean Jadot, one of the most influential figures in shaping American Catholicism in recent years, to a new post at the Vatican.

Jadot, 70, the Vatican's representative in this country for the past seven years, has been named Pro-President of the Vatican Secretariat for Non-Christian Religious, a unit of the Curia, the pope's ruling council. As is customary, the church gave no reasons for the reassignment. Jadot will remain in the delegate post until a successor is named, probably this summer.

During his tenure as apostolic delegate here, Jadot, an appointee of Pope Paul VI, was instrumental in the selection of 174 new bishops for the American church. "Jadot bishops," as they are sometimes called, now comprise about 60 per cent of the American hierarchy.

With few exceptions, they are of the new breed of church leadership. Markedly younger than previous appointees, they are men many church observers see as committed to a progressive and open church that is more involved in community and social justice issues.

Jadot's transfer, which is expected to take place by September, will give Pope John Paul a chance to put his own stamp on relations with the American church. In his nearly two years in office, the Polish pope has followed a more traditional pattern of churchmanship.

The reasons for Jadot's reassignment remained cloaked in the usual Vatican secrecy that surrounds such decisions. An aide to the delegate, asked if Jadot had requested the move, replied, "Not formally that I know . . . these things come in due course."

There is no set term of tenure for the job. "They rotate fairly often," one source said, indicating that Jadot's seven years here was about par for the course.

Although many expressed sadness and disappointment yesterday at Jadot's transfer, it was not wholly unexpected that the present pope would replace the prelate.

One source said that at a private gathering this spring Jadot had expressed interest in another post which might be less demanding than his present one, which entails almost constant travel.

In a formal statement released yesterday, Jadot expressed "regret" at leaving this country but added he was "deeply grateful to Pope John Paul II for giving me the opportunity to serve the church in her ongoing dialogue with all the religions of the world."

In his new post, Jadot succeeds Cardinal Sergio Pignedoli, who died 12 days ago. Pignedoli was considered one of the leading contenders for the papacy when Paul VI died. The appointment to the Secretariat carries with it the virtual certainty of a cardinal's red hat for Jadot.

American churchmen were full of praise yesterday for Jadot. Archbishop John R. Quinn of San Francisco, president of the National Conference of Catholic Bishops, cited Jadot's "keen intelligency, good judgment and vast experience."

Bishop Thomas C. Kelly, general secretary of the conference, said Jadot's efforts had produced "a rich harvest of benefits" for the American church.

A tall smiling man, Jadot is the consummate diplomat who spends much of his time listening. When he speaks, in his French-accented English, the words are chosen carefully, almost delicately, as though to avoid abrasion.

Jadot is not a product of the vaunted Vatican diplomatic service. He served as pastor in various posts in his native Belgium, with time out for a military chaplaincy in the Congo, for 34 years before Pope Paul appointed him in 1968 as apostolic nuncio to Thailand. He also served in Laos, Malaysia and Singapore and in the African nations of Cameroon, Gabon and Equatorial Guinea before his appointment here in 1973.

Jadot, the first non-Italian to represent the church in this country, is himself a new kind of papal envoy. In contrast to his aloof predecessors, he opened the doors of the delegation's mansion opposite the vice presidential residence, on Massachusetts Avenue NW, met with all kinds of lay and clergy groups and readily granted interviews with reporters.

He was particularly concerned with the problems of black and Hispanic groups. During his tenure, nine Hispanic and four black bishops were named in the United States.

"Isn't that a damn shame" exclaimed Robert L. Robinson, a member of the National Black Lay Catholic Caucus, when he learned of Jadot's transfer yesterday. "The black people have lost a friend. He let us know that Rome was very much concerned about the black question."