4 To accomplish this, Christ sent from the Father His Holy Spirit, who was to carry on inwardly His saving work and prompt the Church to spread out. Doubtless, the Holy Spirit was already at work in the world before Christ was glorified. Yet on the day of Pentecost, He came down upon the disciples to remain with them forever (cf. John 14:16). The Church was publicly displayed to the multitude, the Gospel began to spread among the nations by means of preaching, and there was presaged that union of all peoples in the catholicity of the faith by means of the Church of the New Covenant, a Church which speaks all tongues, understands and accepts all tongues in her love, and so supersedes the divisiveness of Babel. For it was from Pentecost that the "Acts of the Apostles" took again, just as Christ was - conceived when the Holy Spirit came upon the Virgin Mary, and just as Christ was impelled to the work of His ministry by the same Holy Spirit descending upon Him while He prayed. Now, the Lord Jesus, before freely giving His life for the world, did so arrange the Apostles' ministry and promise to send the Holy Spirit that both they and the Spirit might be associated in effecting the work of salvation always and everywhere. Throughout all ages, the Holy Spirit makes the entire Church "one in communion and in ministering; He equips her with various gifts of a hierarchical and charismatic nature," a giving life, soul - like, to ecclesiastical institutions and instilling into the hearts of the faithful the same mission spirit which impelled Christ Himself. Sometimes He even visibly anticipates the Apostles' acting, just as He unceasingly accompanies and directs it in different ways.
Source: Ad Gentes (Vatican.va)