Jesus said, "And I have other sheep, that are not of this fold; I must bring them also, and they will heed my voice. So there shall be one flock, one shepherd." Most Catholic biblical scholars, following the teaching of the early Church Fathers, agree that the "other sheep" are the Gentiles, to whom the gospel was sent after the Jews rejected Christ (Rom 11:11-12).
During his public ministry Jesus confined his proclamation of the gospel to the Jews (Mt 10:5-6, 15:24), and initially this remained the focus of the apostles' preaching, although Jesus had foretold that the gospel would eventually be carried to "all nations" (Mt 28:19, Acts 1:8). This opening up of God's blessing even to Gentiles was foretold in the Old Testament (Ps 2:7; Is 2:2-6).
Paul explained this to Gentile Christians:
Therefore remember that at one time you Gentiles in the flesh, called the uncircumcision by what is called the circumcision, which is made in the flesh by hands--remember that you were at that time separated from Christ, alienated from the commonwealth of Israel, and strangers to the covenants of promise, having no hope and without God in the world. But now in Christ Jesus you who once were far off have been brought near in the blood of Christ. (Eph 2:11-13; cf. Rom 3:22; Gal 3:27-28)