CCC 767 “When the work which the Father gave the Son to do on earth was accomplished, the Holy Spirit was sent on the day of Pentecost in order that he might continually sanctify the Church.”1 Then “the Church was openly displayed to the crowds and the spread of the Gospel among the nations, through preaching, was begun.”2 As the “convocation” of all men for salvation, the Church in her very nature is missionary, sent by Christ to all the nations to make disciples of them.3

1 LG 4; cf. Jn 17:4.
2 AG 4.
3 Cf. Mt 28:19-20; AG 2; 5-6.