“Christ bestowed unity on His Church from the beginning. This unity, we believe, subsists in the Catholic Church as something she can never lose, and we hope that it will continue to increase until the end of time. Christ always gives His Church the gift of unity, but the Church must always pray and work to maintain, reinforce, and perfect the unity that Christ will for her…. The desire to recover the unity of all Christians is a gift of Christ and a call of the Holy Spirit” (n. 820).
The shortest route to a proper understanding of ecumenism in the modern context is to read Vatican II’s Decree on Ecumenism, which is a relatively brief and eminently clear document. The principles on which the decree is based are rooted in Scripture, the Fathers of the Church, and ideas set forth in modern times by such popes as Leo XIII and Pius XII.