William Baum, who, as the Catholic archbishop of Washington, welcomed Pope John Paul II to the nation’s capital in 1979 and who became the longest-serving U.S. cardinal in history, died Thursday at a Catholic care facility in the District of Columbia. He was 88.

Baum was archbishop of Washington from 1973 to 1980, during a time of questioning and inner turmoil in the Catholic Church. He was known for initiating dialogue with members of other faiths and for expanding the church’s outreach efforts among minorities.

He was named a cardinal in 1976 by Pope Paul VI and participated in papal conclaves that elected three popes, including two who were close friends: Pope John Paul II and Pope Benedict XVI.

Baum’s 39 years as a cardinal marked the longest tenure of any American Catholic prelate in history, surpassing the 35 years served by James Gibbons, an archbishop of Baltimore, who died in 1921.

With a Protestant father and a Jewish stepfather, Baum embodied an ecumenical outlook from birth. In his 30s, he was already recognized as one of the Catholic Church’s leading authorities on interfaith relations.

During the Second Vatican Council of the 1960s, when the church was reassessing its role and teachings in a rapidly changing world, Baum served as a chief adviser on ecumenical matters to church leaders. He drafted some of the documents on ecumenism for Vatican II.

Throughout his career, he was considered an intellectual force within the church and held powerful positions at the Vatican, where he spent almost as much time as in his native country. He spoke fluent Italian.

Baum lived in Washington from 1964 to 1967, when he was the first executive director of the Bishops’ Commission for Ecumenical and Interreligious Affairs. After serving as a bishop in Missouri, he returned to Washington in 1973, succeeding Patrick O’Boyle as archbishop. At the time, the archdiocese was home to about 390,000 Catholics.

In his first homily as archbishop, Baum declared that the Catholic Church “clearly must do battle with racism and with all other forces which threaten human life and liberty.”

He described racial prejudice as a “heresy and sin” and in 1974 established a Black Secretariat, or a council of clergy members and lay people designed to give African-Americans a greater voice in the church.