A 48-year-old priest who holds a high administrative position in the Roman Catholic Archdiocese of Chicago, was named bishop of Arlington yesterday.

The Very Rev. John Richard Keating, vicar general and chancellor of the Chicago archdiocese, was named by the Vatican to succeed the conservative Bishop Thomas J. Welsh, who was transferred in February to the Diocese of Allentown, Pa.

Keating will be consecrated a bishop and installed as the second head of the nine-year-old, 188,000-member Northern Virginia diocese on Aug. 4.

He is regarded as progressive in the church matters and was given high marks by Chicago priests and church officials for his administrative skills and sensitive handling of his duties during the troubled reign of the late Cardinal John P. Cody.

"One of our finest," was the way the Rev. James Roach, administrative assistant to Cardinal Joseph L. Bernardin, described Keating. "Very bright, very sensitive, an excellent priest. He's highly respected by the priests and the people of the Chicago Archdiocese as really fair-minded. He will be an excellent bishop," said Roach.

Bernardin, Cody's successor, praised Keating's "deep faith and commitment to the church, intelligence, personal charm and an extraordinary sense of fairness." The people of Northern Virginia, he continued "will be welcoming as their new shepherd a man who has a remarkable vision of the church and who will be very sensitive to their needs and aspirations."

Keating comes into a young diocese that has become known for controversies between the conservative founding bishop, Welsh, and several more progressive parishes. The new bishop, in a telephone interview, sidestepped questions about his liberal or conservative sympathies by saying: "I hope I will be able to carry . . . a sense of fairness to Arlington, to be able to sense the needs of all."

Archbishop James A. Hickey of Washington, who said he has served with Keating "on several projects," welcomed the Chicagoan "as a friend of some years and as a new neighbor."

Msgr. John Hannan, who has been serving as administrator of the Arlington diocese, "joyfully" welcomed the appointment and pledged "our reverence, obedience and cooperation."

Keating is well known in the church for the impact of his doctoral dissertation on church annulments. His study, "The Bearing of Mental Impairment on the Validity of Marriage," completed in l963 and published the following year, laid the groundwork for church tribunals around the world to annul marriages on grounds of psychological flaws in one or both spouses at the time of the marriage.

The overwhelming majority of annulments, which permit partners in the failed marriage to remarry with the church's blessing, are based on psychological grounds.

Msgr. Daniel Hoye, general secretary of the National Conference of Catholic Bishops, characterized Keating's work as "the classic" and "one of the turning points" for the development of psychological grounds for annulment.

The Arlington diocese is made up of the 21 Virginia counties and cities as far south as Fredericksburg. It was spun off from the Diocese of Richmond nearly nine years ago when the rapid growth of the church in the Washington suburbs made the entire state too unwieldy to be administered as one diocese.

Keating is a Chicago native. He was a classmate of former Mayor Jane Byrne at the Queen of All Saints Parochial School, where he was once cast as the prince in a school play.

His record at archdiocesan seminaries caught the eye of the late Cardinal Albert Meyer who sent him to Rome to study theology and canon law at the Gregorian University. He was ordained in Rome in 1958.

The Rev. Charles Curran, who teaches moral theology at Catholic University and who was a classmate of Keating in Rome, recalls him as "very caring, very bright, very active, very prudent." He had the marks even then, said Curran, of a man "highly likely to become a bishop."

In 1963, Keating returned to Chicago as assistant chancellor. In 1971, he was named cochancellor with responsibilities for priest personnel. In the nation's largest archdiocese with 2,500 priests, it would have been a critical assignment in the best of circumstances.

The post-Vatican II ferment in the church worldwide was at its height, with substantial numbers of men leaving the priesthood. The conflict was exacerbated in Chicago where the autocratic Cody steadfastly refused to implement the Vatican Council's vision of shared responsibility in the church.

Keating managed to retain the confidence of both his fellow priests and Cody.

In 1979, he was named vicar general and chancellor, posts that he held until Cody's death in April of last year. When Cody became seriously ill, observers generally concede, Keating virtually ran the archdiocese.

On Cody's death, he was elected by the 12-man Board of Consulters of the diocese as administrator. When Bernardin became archbishop last summer, he kept Keating as vicar general and chancellor.