Marking a generational change for one of the Catholic Church’s most influential organizations, Spanish Bishop Javier Echevarría Rodríguez, the leader of Opus Dei, died in Rome on Monday at the age of 84.

Echevarría had been hospitalized in Rome’s Campus Biomedico on Dec. 5, and died of complications due to a pulmonary infection. He marks the last close personal aide to the group’s founder to succeed him as its leader.

Pope Francis on Tuesday expressed condolences, while also giving thanks for Echevarría’s “generous witness of priestly and episcopal life” and his “constant service of love to the Church and for souls.”

Echevarría had been appointed the prelate of Opus Dei under St. Pope John Paul II in 1994, and was named a bishop by John Paul one year later in 1995. Under the Code of Canon Law, Opus Dei is considered a “personal prelature,” a structure which unites a prelate, clergy and laity in specific pastoral activities, which are defined not by geography like a diocese but a chosen spiritual path regardless of where members live.

To date, Opus Dei remains the lone personal prelature in the Catholic Church, assigned that status in 1982 by John Paul II. In 2015 its membership was roughly 94,000 people worldwide, including over 91,000 laity and more than 2,000 priests.

Founded in 1928 by Saint Josemaría Escrivá, Opus Dei is a body that was for many years the subject of considerable internal controversy.

Seen by many observers as a largely conservative force within Catholicism, Opus Dei has been criticized among other things for its alleged wealth, its emphasis on papal loyalty and obedience, its attitudes towards women, and its practice of “corporal mortification,” including use of the cilice, a small barbed chain worn by core members around the upper thigh.

On the other hand, admirers regard Opus Dei as a highly committed corps of mostly lay Catholics who undergo a serious program of spiritual and theological formation and who generally are willing to make a profound commitment to the Catholic Church and to the faith.

Famously, Opus Dei was featured in the 2003 Dan Brown novel The Da Vinci Code as a nefarious force within the Catholic Church, though in the years since, much of the controversy surrounding the organization has largely abated.

Despite its relatively small size, Opus Dei is considered an influential force within Catholicism given the prominence of many of its members and the favor with which many bishops and other Catholic leaders see the group.

Echevarría represented the last of the original generation of Opus Dei figures to head the organization. Escrivá himself held that role until his death in 1975, and was succeeded by his close aide Bishop Álvaro del Portillo, who led the organization until 1994 and is today considered “Blessed” by the Catholic Church, meaning the penultimate stage before sainthood.

Portillo was beatified in September 2014 with the permission of Pope Francis, who knew Opus Dei and formed friendships with several members of the group during his time in Argentina.

Like Portillo, Echevarría had been a personal confidante to Escrivá, serving as his personal secretary from 1953 to 1975. He became the Secretary General of Opus Dei in 1975, and the Vicar General in 1982 upon Escrivá’s death.

According to a statement released by Opus Dei on Monday, Echevarría will be replaced for the moment by Spanish Monsignor Fernando Ocariz, the current Vicar General, who assumed that office in 1994. According to Opus Dei’s statutes, Ocariz has one month to call a “collective congress” to choose his replacement, which should take place within three months.

According to the group’s statutes, whoever is selected will need to be approved by the pope.

Also according to the Opus Dei statement, Ocariz administered the last rites of the Church to Echevarría before he died.

By papal appointment, Echevarría had served as a member of both the Congregation for the Causes of Saints and the Apostolic Signatura, in effect the Vatican’s Supreme Court. He was the author of five books, including a set of memoirs of Escrivá.

In general, Echevarría was regarded in Rome as a personally pious and non-flamboyant personality, who helped steer Opus Dei toward a period of normalization after the controversies surrounding the canonization of Escrivá in 2002 and then the turbulence of the Da Vinci Code.