15 Passing from principles, from the obligations of the Christian conscience, to the actual practice of the ecumenical journey towards unity, the Second Vatican Council emphasizes above all the need for interior conversion . The messianic proclamation that "the time is fulfilled and the Kingdom of God is at hand", and the subsequent call to "repent, and believe in the Gospel" ( Mk 1:15) with which Jesus begins his mission, indicate the essential element of every new beginning: the fundamental need for evangelization at every stage of the Church's journey of salvation. This is true in a special way of the process begun by the Second Vatican Council, when it indicated as a dimension of renewal the ecumenical task of uniting divided Christians. "There can be no ecumenism worthy of the name without a change of heart ". The Council calls for personal conversion as well as for communal conversion. The desire of every Christian Community for unity goes hand in hand with its fidelity to the Gospel. In the case of individuals who live their Christian vocation, the Council speaks of interior conversion, of a renewal of mind. Each one therefore ought to be more radically converted to the Gospel and, without ever losing sight of God's plan, change his or her way of looking at things. Thanks to ecumenism, our contemplation of "the mighty works of God" ( mirabilia Dei ) has been enriched by new horizons, for which the Triune God calls us to give thanks: the knowledge that the Spirit is at work in other Christian Communities, the discovery of examples of holiness, the experience of the immense riches present in the communion of saints, and contact with unexpected dimensions of Christian commitment. In a corresponding way, there is an increased sense of the need for repentance: an awareness of certain exclusions which seriously harm fraternal charity, of certain refusals to forgive, of a certain pride, of an unevangelical insistence on condemning the "other side", of a disdain born of an unhealthy presumption. Thus, the entire life of Christians is marked by a concern for ecumenism; and they are called to let themselves be shaped, as it were, by that concern.
Source: Ut Unum Sint (Vatican.va)