The Most Rev. John Richard Keating, a church administrator for most of his 25 years in the priesthood, was consecrated yesterday as the second Roman Catholic bishop of Arlington, a fast-growing diocese that stretches over Northern Virginia and includes 188,000 church members.

More than a score of bishops and two cardinals joined the Vatican's Apostolic Delegate, Archbishop Pio Laghi, in the solemn laying-on of hands that made Keating a bishop. The rite, reenacted at the Cathedral of St. Thomas More in Arlington, has signaled the passing of consecrated leadership of the Roman Catholic Church in an unbroken succession from Christ and his first apostles.

It was the high point of a solemn and dramatic two-hour ritual in which Msgr. William T. Reinecke, chancellor of the Arlington diocese, read the formal mandate of the Vatican secretary of State, Cardinal Augostino Caseroli, designating Keating, a 49-year-old Chicago-born priest "to be the good shepherd of the church of Arlington."

The centuries-old consecration rite was reenacted before a hushed congregation of some 1,400. After each bishop present had, in turn, placed his hands on the bowed head of the kneeling Keating, the new bishop was helped to his feet and given the symbols of his office: the bishop's ring, miter, and crozier.

Keating was greeted with enthusiastic applause as he faced his flock. In the front row sat his parents, Mr. and Mrs. Robert Keating of Sun City, Ariz., his father beaming proudly and his mother wiping away tears.

Moving to the pulpit, Keating spoke with warmth and humor. "People here present from the archdiocese of Chicago have been hearing me talk for 25 years," he said. "People of the diocese of Arlington will be hearing me talk for the next 25. Mercy dictates that I be brief today."

He paid tribute to his predecessor, Bishop Thomas J. Welsh, now bishop of Allentown and the only bishop the nine-year-old diocese has had. Welsh was "a remarkable proponent of Christian education, of human life, of religious vocations," Keating said. "I am truly honored to be called to follow in your footsteps."

He also praised Msgr. John P. Hamman who has been serving as diocesan administrator. "I can relate to what you have been going through" since 15 months ago he had a similar task in the archdiocese of Chicago in the interim between the death of Cardinal John Cody and the installation of Cardinal Joseph L. Bernardin.

Bernardin, who was chosen by Keating to give the homily at yesterday's ceremony, said that "we describe the office of bishop primarily in terms of ministry or service." And the bishop's authority, he said, "should be exercised as a service and with the wide latitude that would be expected of a good shepherd who knows how to evaluate time and persons, potentials and limitations, frailty and talent."

At times, Bernardin said, authority "is exercised more through our caring presence than through counsel and directives--a presence which demonstrates our love and concern for people even when we may not have an immediate solution for their problem."

Yesterday's ceremony combined the installation service with the rite of elevating Keating to the rank of bishop. The new prelate has spent virtually all of his professional career in church administration, much of the time at the eye of the storms that have swept American Catholicism in the last 20 years.

His doctoral dissertation, "The Bearing of Mental Impairment on the Validity of Marriage," is recognized by church scholars as a foundation for the church's attitude toward marriage. It provided the basis for church tribunals to annul marriages on the grounds of psychological flaws in one or both spouses at the time of the marriage and has made it possible for thousands of divorced Catholics to remarry with the church's blessing.

From 1963 until his appointment in June as bishop of Arlington, Keating helped administer the massive Chicago archdiocese, first under Cardinal Albert Meyer but since 1965, under the strife-torn regime of the late Cardinal John Cody. For much of that time, Keating's work included counseling and processing the scores of Chicago priests leaving the priesthood.

When Cody died in April 1982, Keating administered the archdiocese for five months until Bernardin took over.

The Arlington diocese which Keating heads covers 21 Virginia counties, from Arlington south to Fredericksburg and Kilmarnock on the Northern Neck.