There can be no ecumenism worthy of the name without a change of heart... 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…
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.
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."
Second Vatican Council
Unitatis redintegratio 7-8