20 All this is extremely important and of fundamental significance for ecumenical activity. Thus it is absolutely clear that ecumenism, the movement promoting Christian unity, is not just some sort of "appendix " which is added to the Church's traditional activity. Rather, ecumenism is an organic part of her life and work, and consequently must pervade all that she is and does; it must be like the fruit borne by a healthy and flourishing tree which grows to its full stature. This is what Pope John XXIII believed about the unity of the Church and how he saw full Christian unity. With regard to other Christians, to the great Christian family, he observed: "What unites us is much greater than what divides us". The Second Vatican Council for its part exhorts "all Christ's faithful to remember that the more purely they strive to live according to the Gospel, the more they are fostering and even practising Christian unity. For they can achieve depth and ease in strengthening mutual brotherhood to the degree that they enjoy profound communion with the Father, the Word, and the Holy Spirit".
Source: Ut Unum Sint (Vatican.va)