26 Prayer, the community at prayer, enables us always to discover anew the evangelical truth of the words: "You have one Father " (Mt 23:9), the Father —Abba —invoked by Christ himself, the Only-begotten and Consubstantial Son. And again: "You have one teacher , and you are all brethren " ( Mt 23:8). "Ecumenical" prayer discloses this fundamental dimension of brotherhood in Christ, who died to gather together the children of God who were scattered, so that in becoming "sons and daughters in the Son" (cf. Eph 1:5) we might show forth more fully both the mysterious reality of God's fatherhood and the truth about the human nature shared by each and every individual. "Ecumenical" prayer, as the prayer of brothers and sisters, expresses all this. Precisely because they are separated from one another, they meet in Christ with all the more hope, entrusting to him the future of their unity and their communion . Here too we can appropriately apply the teaching of the Council: "The Lord Jesus, when he prayed to the Father 'that all may be one ... as we are one ' ( Jn 17:21-22), opened up vistas closed to human reason. For he implied a certain likeness between the union of the Divine Persons, and the union of God's children in truth and charity". The change of heart which is the essential condition for every authentic search for unity flows from prayer and its realization is guided by prayer: "For it is from newness of attitudes, from self-denial and unstinted love, that yearnings for unity take their rise and grow towards maturity. We should therefore pray to the divine 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".
Source: Ut Unum Sint (Vatican.va)