During his pontificate, St. John XXIII was a big proponent of Church unity. He wrote an entire encyclical on the topic entitled Ad petri cathedram, which addressed unity not only among Christians in general, but also within the Catholic Church.
Above all else he believed prayer was essential to establish unity, echoing Jesus’ own prayer that “they may all be one, as you, Father, are in me and I in you, that they also may be in us, that the world may believe that you sent me” (John 17:21).
St. John XXIII went so far as to call for a “crusade of prayer” for unity. The context of his “crusade” was the upcoming Second Vatican Council, and while that event has already passed, we can affirm that the unity of the Church in the 21st century, “will depend more on a crusade of fervent prayer than on human effort and diligent application.”
Here are the words of his exhortation to prayer, something that we should all meditate on and consider in our own lives, doing all that we can for unity within the Catholic Church.
We address suppliant prayers to our gracious God, the giver of heavenly light and of all good things, that He safeguard the unity of the Church and extend the fold and kingdom of Christ. We urge all Our brethren in Christ and Our beloved sons to pray fervently for the same intentions. The outcome of the approaching Ecumenical Council will depend more on a crusade of fervent prayer than on human effort and diligent application. And so with loving heart We also invite to this crusade all who are not of this fold but reverence and worship God and strive in good faith to obey His commands.May the divine plea of Christ further and fulfill this hope and these prayers of Ours: “Holy Father, keep in thy name those whom thou hast given me, that they may be one even as we are … Sanctify them in the truth. Thy word is truth … Yet not for these only do I pray, but for those also who through their word are to believe in m … that they may be perfected in unity …”We repeat this prayer, as does the whole Catholic world in union with Us. We are spurred by a burning love for all men, but also by that interior humility which the gospel teaches. For We know the lowliness of him whom God raised to the dignity of the Sovereign Pontificate, not because of Our merits, but according to His mysterious designs. Wherefore, to all Our brethren and sons who are separated from the Chair of Blessed Peter, We say again: “I am . . . Joseph, your brother.” Come, “make room for us.” (41) We want nothing else, desire nothing else, pray God for nothing else but your salvation, your eternal happiness.
Prayer for unity in the Church and among Christians by John Paul II