23 Finally, fellowship in prayer leads people to look at the Church and Christianity in a new way . It must not be forgotten in fact that the Lord prayed to the Father that his disciples might be one, so that their unity might bear witness to his mission and the world would believe that the Father had sent him (cf. Jn 17:21). It can be said that the ecumenical movement in a certain sense was born out of the negative experience of each one of those who, in proclaiming the one Gospel, appealed to his own Church or Ecclesial Community. This was a contradiction which could not escape those who listened to the message of salvation and found in this fact an obstacle to acceptance of the Gospel. Regrettably, this grave obstacle has not been overcome. It is true that we are not yet in full communion. And yet, despite our divisions, we are on the way towards full unity, that unity which marked the Apostolic Church at its birth and which we sincerely seek. Our common prayer, inspired by faith, is proof of this. In that prayer, we gather together in the name of Christ who is One. He is our unity. "Ecumenical" prayer is at the service of the Christian mission and its credibility . It must thus be especially present in the life of the Church and in every activity aimed at fostering Christian unity. It is as if we constantly need to go back and meet in the Upper Room of Holy Thursday, even though our presence together in that place will not be perfect until the obstacles to full ecclesial communion are overcome and all Christians can gather together in the common celebration of the Eucharist.
Source: Ut Unum Sint (Vatican.va)