What reforms within the Roman Catholic Church during the era of the Protestant Reformation were expressed principally in council?

Answer 1

Roman Catholic AnswerThe Council of Trent, which was the beginning of the Church's response to the protestant revolt was probably the most conservative of the ecumenical councils of the Church. It addressed no new issues, but simply reiterated what had always been taught in the Church, it restated previously held beliefs and spelled out in great detail things which, perhaps, had not been so explained before. See the link below the answer for a complete list of the decrees of the Council of Trent.

from A Catholic Dictionary, edited by Donald Attwater, Second edition, revised 1957

The Council of Trent

The 19th ecumenical council held at Trent in the Austrian Tyrol, 1545-63, summoned for the purpose of combating Protestantism and reforming the discipline of the Church; the longest and one of the most important of all general councils. It dealt in detail with the doctrinal innovations of the Reformers and with those gross abuses which gave them an opportunity to take root. It was one of the most important events of modern history and has had lasting effect. The principal dogmatic decisions were: the confirmation of the Nicene creed; the authenticity of the Latin Vulgate and the canonicity of all books contained therein and of them only; the definition of the doctrine of Original Sin; the precision of the doctrine of Justification, condemning justification by faith alone and imputation of grace; the condemnation of thirty errors about the sacraments; the definition of the Real Presence and of Transubstantiation as its mode: the precision of the doctrine of the sacraments of penance and Extreme Unction; the declaration that holy communion in both kinds was not necessary for lay-people and clerics not celebrating, Christ being received whole and entire under either species; the precision of doctrine concerning the sacrifice of the Mass and the sacraments of holy Orders and Matrimony; the affirmation of the doctrines of Purgatory, of the invocation of saints, and the veneration of them, their relics and images, and of Indulgences. Far-reaching decrees of reformation in discipline and morals were adopted involving many alterations in canon law, e.g. the decree Tametsi.