After knocking three times on the massive front door, the Right Rev. Ronald H. Haines entered the Washington Cathedral last night and was installed as the seventh bishop of the Episcopal Diocese of Washington.

Haines, who became acting bishop of the 39,000-member diocese after the death of Bishop John Walker a year ago, was elected to the post in June. His election then was confirmed by a majority of U.S. bishops and dioceses.

At 55, Haines assumes responsibility for one of the most visible Episcopal dioceses in the country. He also will serve as chief executive officer of the Protestant Episcopal Cathedral Foundation, which runs the cathedral and several schools.

In a sermon after his installation, Haines indicated that he, like Walker, will not shy away from taking political stands. Drawing on the example of the Biblical prophet Isaiah, who became leader of the nation of Israel at a time of political and military unrest, Haines said, "When Isaiah accepted the call {from God}, he stepped into the arena of politics as well as religion."

The new bishop lamented the fact that the United States is poised for war in the Mideast. "It is sobering to reflect that the money spent on one Stealth bomber, minus hardware, would build five cathedrals such as this," he said in the sermon. "It is imperative that people of faith seek those priorities that lead society toward justice and peace."

Haines, echoing a pledge he had made while campaigning for the bishop's position, said he will concentrate on the needs of the diocese's 93 congregations in the District and suburban Maryland. The diocese of Virginia is separate.

One of those tasks will be to address the diocese's financial problems, something that Haines, who spent several years in business before going to seminary, apparently is comfortable with. Haines said last night that he will encourage all Episcopalians to tithe 10 percent of their income to the church.

Haines said he will also encourage his flock to spend time each week in prayer and reading the Bible. He said the church, under his leadership, will continue to reach out to other faiths and to put into positions of power and influence representatives of diverse groups, a reference to women and minorities.

Haines, who is married and has six children, served his first church in 1967 in the Diocese of Long Island. He moved from there to the Bronx and then to Rutherfordton, N.C., where he served for 13 years.

He was a deputy bishop in North Carolina before being elected assistant bishop here in 1986. As Walker's assistant, Haines traveled the diocese extensively as a sort of trouble-shooter and became well-known to many congregations.

About 1,200 people attended last night's two-hour installation ceremony, which was presided over by the Most Rev. Edmond Browning, the presiding bishop of the Episcopal Church in the United States. The Rev. Sanford Garner, interim provost at Washington National Cathedral, read a letter of congratulation to Haines from President Bush, an Episcopalian.