“…The attainment of union is the concern of the whole Church, faithful and shepherds alike. This concern extends to everyone, according to his talent, whether it be exercised in his daily Christian life or in his theological and historical research. This concern itself reveals already to some extent the bond of brotherhood between all Christians and it helps toward that full and perfect unity which God in His kindness wills.
Every renewal of the Church is essentially grounded in an increase of fidelity to her own calling. Undoubtedly this is the basis of the movement toward unity.
Christ summons the Church to continual reformation as she sojourns here on earth. The Church is always in need of this, in so far as she is an institution of men here on earth. Thus if, in various times and circumstances, there have been deficiencies in moral conduct or in church discipline, or even in the way that church teaching has been formulated-to be carefully distinguished from the deposit of faith itself-these can and should be set right at the opportune moment.
Church renewal has therefore notable ecumenical importance. Already in various spheres of the Church's life, this renewal is taking place. The Biblical and liturgical movements, the preaching of the word of God and catechetics, the apostolate of the laity, new forms of religious life and the spirituality of married life, and the Church's social teaching and activity-all these should be considered as pledges and signs of the future progress of ecumenism.
There can be no ecumenism worthy of the name without a change of heart. For it is from renewal of the inner life of our minds, from self-denial and an unstinted love that desires of unity take their rise and develop in a mature way. We should, therefore, pray to the Holy Spirit for the grace to be genuinely self-denying, humble. gentle in the service of others, and to have an attitude of brotherly generosity towards them. St. Paul says: "I, therefore, a prisoner for the Lord, beg you to lead a life worthy of the calling to which you have been called, with all humility and meekness, with patience, forbearing one another in love, eager to maintain the unity of the spirit in the bond of peace". This exhortation is directed especially to those raised to sacred Orders precisely that the work of Christ may be continued. He came among us "not to be served but to serve".
The words of St. John hold good about sins against unity: "If we say we have not sinned, we make him a liar, and his word is not in us". So we humbly beg pardon of God and of our separated brethren, just as we forgive them that trespass against us.
All the faithful should remember that the more effort they make to live holier lives according to the Gospel, the better will they further Christian unity and put it into practice. For the closer their union with the Father, the Word, and the Spirit, the more deeply and easily will they be able to grow in mutual brotherly love.
This change of heart and holiness of life, along with public and private prayer for the unity of Christians, should be regarded as the soul of the whole ecumenical movement, and merits the name, "spiritual ecumenism."
It is a recognized custom for Catholics to have frequent recourse to that prayer for the unity of the Church which the Savior Himself on the eve of His death so fervently appealed to His Father: "That they may all be one".
In certain special circumstances, such as the prescribed prayers "for unity," and during ecumenical gatherings, it is allowable, indeed desirable that Catholics should join in prayer with their separated brethren. Such prayers in common are certainly an effective means of obtaining the grace of unity, and they are a true expression of the ties, which still bind Catholics to their separated brethren. "For where two or three are gathered together in my name, there am I in the midst of them".
Yet worship in common (communicatio in sacris) is not to be considered as a means to be used indiscriminately for the restoration of Christian unity. There are two main principles governing the practice of such common worship: first, the bearing witness to the unity of the Church, and second, the sharing in the means of grace. Witness to the unity of the Church very generally forbids common worship to Christians, but the grace to be had from it sometimes commends this practice…”
Cf. Ecumenical Decree “UNITATIS REDINTEGRATIO – PP. Paul VI