When Jesus spoke of "one flock, one shepherd," did he mean the Catholic Church alone?
The short answer
The Church teaches that the "one flock, one shepherd" subsists in the Catholic Church, governed by Peter's successor and the bishops in communion with him. Many truths are found elsewhere, but the fullness of Christ's teaching and grace is in the Catholic Church, whose mission is to draw all humanity into unity under Christ.
Subsists in the Catholic Church
The "one flock, one shepherd" of John 10:16 subsists in the Catholic Church, governed by Peter's successor and the bishops in communion with him (Dominus Iesus 17).
Fullness
Many truths are found in other communities, but the full teaching and grace of Christ is found only in the Catholic Church. That is why Jesus prayed for his disciples' unity, that they may all be one (John 17:21; CCC 820).
The Church's mission
Her mission is to work ceaselessly for the return of all humanity under Christ the Head in the unity of his Spirit (CCC 831). With Benedict XVI we pray: Lord, grant that we may be one flock and one shepherd.