5 Together with all Christ's disciples, the Catholic Church bases upon God's plan her ecumenical commitment to gather all Christians into unity. Indeed, "the Church is not a reality closed in on herself. Rather, she is permanently open to missionary and ecumenical endeavour, for she is sent to the world to announce and witness, to make present and spread the mystery of communion which is essential to her, and to gather all people and all things into Christ, so as to be for all an 'inseparable sacrament of unity' ". Already in the Old Testament, the Prophet Ezekiel, referring to the situation of God's People at that time, and using the simple sign of two broken sticks which are first divided and then joined together, expressed the divine will to "gather from all sides" the members of his scattered people. "I will be their God, and they shall be my people. Then the nations will know that I the Lord sanctify Israel" (cf. 37:16-28). The Gospel of John, for its part, considering the situation of the People of God at the time it was written, sees in Jesus' death the reason for the unity of God's children: "Jesus would die for the nation, and not for the nation only, but to gather into one the children of God who are scattered abroad" (11:51-52). Indeed, as the Letter to the Ephesians explains, Jesus "broke down the dividing wall of hostility ... through the Cross, thereby bringing the hostility to an end"; in place of what was divided he brought about unity (cf. 2:14-16).
Source: Ut Unum Sint (Vatican.va)