The Church teaches that the “one flock, one shepherd” in John 10:16 subsists in the Catholic Church, governed by the successor of Peter and by the bishops in communion with him (Dominus Iesus 17).
While the Church acknowledges that many truths are found in other ecclesial communities, the complete fullness of Christ’s teaching and grace is only found in the Catholic Church:
This is why Jesus himself prayed at the hour of his Passion, and does not cease praying to his Father, for the unity of his disciples: “That they may all be one. As you, Father, are in me and I am in you, may they also be one in us, . . . so that the world may know that you have sent me.” (John 17:21, CCC 820)
To work ceaselessly and efficaciously for the return of all humanity and all its goods, under Christ the Head in the unity of his Spirit is the mission of the Church (CCC 831).
With Pope Benedict VXI, “Let us remember it in our prayer to the Lord, as we plead with him: yes, Lord, remember your promise. Grant that we may be one flock and one shepherd! Do not allow your net to be torn, help us to be servants of unity!” (Homily of His Holiness Benedict XVI, St. Peter’s Square, April 24, 2005)
Further reading: Catechism of the Catholic Church, 11-870; One, Holy, Catholic and Apostolic: The Early Church Was the Catholic Church by Kenneth D. Whitehead