He is one of two cardinals – along with Juan Luis Cipriani Thorne – who belong to Opus Dei; Herranz Casado is the organisation's highest-ranking member in the Church's hierarchy. He is also considered one of the foremost experts in canon law, and to have been one of the Vatican's most influential figures during the period shortly before the death of Pope John Paul II.
According to Vatican journalist Sandro Magister, by the end of 2004, Herranz Casado was "constantly gaining influence" in the internal affairs of the Vatican.[citation needed] Along with Joseph Ratzinger, Angelo Sodano, and the Pope's private secretary, Archbishop Stanisław Dziwisz, Herranz Casado is believed to have been largely responsible for leading the Curia at times when the Pope was incapacitated by illness.[5] Herranz finds conspiracy theories about Opus Dei particularly offensive, claiming that it has "no hidden agenda. The only policy is the message of Christ".[6]
Upon the death of John Paul II on 2 April 2005, Herranz Casado and all major Vatican officials automatically lost their positions. Herranz Casado was confirmed as president of Legislative Texts by Pope Benedict XVI on the following 21 April.[7] He was one of the cardinal electors in the 2005 papal conclave. Though not generally considered a strong candidate for the papacy himself, he was described as a highly influential insider with the potential to play the role of a "kingmaker" at the conclave. It has been reported that, both before and after Pope John Paul's death, Herranz convened meetings of cardinals at a villa in Grottarossa, a suburb of Rome.[8]Pope Benedict XVI named Francesco Coccopalmerio to replace Herranz as President of the Pontifical Council for Legislative Texts on 15 February 2007.[9]
In March 2012 Pope Benedict XVI established a Commission of Cardinals to investigate leaks of reserved and confidential documents on television, in newspapers, and in other communications media (in what is known as the Vatileaks scandal). It first met on Tuesday, 24 April 2012. Herranz served as the chair, and was accompanied by Cardinals Jozef Tomko and Salvatore De Giorgi.[10]
Having been a cardinal-deacon for ten years, he was promoted to cardinal-priest by Pope Francis on 12 June 2014.[11]