The Catechism in Questions & Answers

583. How did the gospel come to the Gentiles?

The Apostles at first worked under the assumption that the gospel was only to be preached to the Jews. In a vision, however, God made it clear to Apostle Peter that the gospel is also intended for the Gentiles (cf. Acts 10 and 11).
At the Apostles’ council in Jerusalem, questions concerning the mission to the Gentiles and the significance of the Mosaic Law for baptised Gentiles were clarified (cf. Acts 15: 1-29). These decisions were contributing factors in helping the Christian congregations to eventually shed many traditions of the Jewish faith.

Gentiles: see explanation of Question 256. Mosaic Law: see Questions 272. et seq.

In special cases, God uses a “vision” to reveal His will to human beings whom He chooses for this purpose.