Which council was significant in defining Catholic beliefs and responding to the Protestant Reformation?

The Council of Trent was pivotal in articulating and solidifying Catholic doctrine in response to the challenges posed by the Protestant Reformation. Convened in stages between 1545 and 1563, this council addressed key issues raised by reformers like Martin Luther and John Calvin. It reaffirmed central Catholic tenets such as the authority of the Pope, the importance of traditions alongside Scripture, the necessity of sacraments, and the role of good works in salvation.

By doing so, the Council of Trent not only established a clear counter-narrative to Protestant beliefs but also set in motion a series of reforms within the Catholic Church, leading to significant changes in practices, discipline, and the education of clergy. This resulted in a more unified Catholic identity that aimed to respond to the theological and social upheaval of the time.

In contrast, the other councils mentioned—Nicaea, Constantinople, and Florence—addressed a variety of other theological debates but were not focused specifically on the issues of the Protestant Reformation nor had the same lasting impact on Catholic doctrine in that historical context. Therefore, the Council of Trent stands out as the foundational assembly in defining Catholic beliefs during the tumultuous era of the Reformation.