Peritus (Latin for "expert") is the title given to Roman Catholic theologians attending an ecumenical council to give advice. At the Second Vatican Council, some periti (the plural form) accompanied individual bishops or groups of bishops from various countries. Others were formally appointed as advisers to the whole Council.
Fr. Joseph Ratzinger who became Pope Benedict XVI served as peritus to Cardinal Josef Frings, Archbishop of Cologne, Germany, while Hans Küng was a peritus for the Second Vatican Council, rather than for an individual Bishop.
Cardinal Yves Congar, O.P. served as a consultant to the Council upon the invitation of Pope John XXIII, but was hired as personal and expert theologian (peritus) at the council to Bishop Jean-Julien Weber of Strasbourg which allowed him to attend all the general sessions and to take participate in discussions of any commission to which he was invited to.
The periti often advocated ideas of reform[dubious ] in the Church and were often at the center of debates with some of the more traditional scholars from the Coetus Internationalis Patrum.[according to whom?]