What was the primary decree issued at the Council of Vatican II regarding ecumenism?

The primary decree issued at the Second Vatican Council regarding ecumenism is indeed "Unitatis Redintegratio." This document specifically addressed the importance of promoting unity among Christian denominations and emphasized the church's commitment to ecumenism, which seeks to foster understanding and cooperation between different Christian traditions.

"Unitatis Redintegratio" articulates the Catholic Church’s perspective on the necessity of dialogue with other traditions, acknowledging the validity of elements of truth present in other churches and communities. It encourages a spirit of openness and collaboration, urging Catholics to engage constructively with fellow Christians in pursuit of fuller unity. By promoting this decree, the Council aimed to reconcile divisions and create a more inclusive Christian witness in the modern world.

The other options, while significant in their own right, do not specifically focus on ecumenism. The decree on liturgy addresses reforms in the church’s liturgical practices, the decree on religious freedom speaks to issues of conscience and the rights of individuals regarding faith, and the decree on priestly formation details the education and training of priests. Each of these documents contributes to the broader mission of the Church, but "Unitatis Redintegratio" is distinctly centered on the theme of ecumenical dialogue and unity among Christians.