CCC 244 The eternal origin of the Holy Spirit is revealed in his mission in time. The Spirit is sent to the apostles and to the Church both by the Father in the name of the Son, and by the Son in person, once he had returned to the Father.1 The sending of the person of the Spirit after Jesus’ glorification2 reveals in its fullness the mystery of the Holy Trinity.

CCC 260 The ultimate end of the whole divine economy is the entry of God’s creatures into the perfect unity of the Blessed Trinity.3 But even now we are called to be a dwelling for the Most Holy Trinity: “If a man loves me”, says the Lord, “he will keep my word, and my Father will love him, and we will come to him, and make our home with him”:4
O my God, Trinity whom I adore, help me forget myself entirely so to establish myself in you, unmovable and peaceful as if my soul were already in eternity. May nothing be able to trouble my peace or make me leave you, O my unchanging God, but may each minute bring me more deeply into your mystery! Grant my soul peace. Make it your heaven, your beloved dwelling and the place of your rest. May I never abandon you there, but may I be there, whole and entire, completely vigilant in my faith, entirely adoring, and wholly given over to your creative action.5

CCC 1099 The Spirit and the Church cooperate to manifest Christ and his work of salvation in the liturgy. Primarily in the Eucharist, and by analogy in the other sacraments, the liturgy is the memorial of the mystery of salvation. The Holy Spirit is the Church’s living memory.6

CCC 2615 Even more, what the Father gives us when our prayer is united with that of Jesus is “another Counselor, to be with you for ever, even the Spirit of truth.”7 This new dimension of prayer and of its circumstances is displayed throughout the farewell discourse.8 In the Holy Spirit, Christian prayer is a communion of love with the Father, not only through Christ but also in him: “Hitherto you have asked nothing in my name; ask, and you will receive, that your joy may be full.”9

CCC 2623 On the day of Pentecost, the Spirit of the Promise was poured out on the disciples, gathered “together in one place.”10 While awaiting the Spirit, “all these with one accord devoted themselves to prayer.”11 The Spirit who teaches the Church and recalls for her everything that Jesus said12 was also to form her in the life of prayer.

1 Cf. Jn 14:26; 15:26; 16:14.
2 Cf. Jn 7:39.
3 Cf. Jn 17:21-23.
4 Jn 14:23.
5 Prayer of Blessed Elizabeth of the Trinity.
6 Cf. Jn 14:26.
7 Jn 14:16-17.
8 Cf. Jn 14:23-26; 15:7, 16; 16:13-15; 16:23-27.
9 Jn 16:24.
10 Acts 2:1.
11 Acts 1:14.
12 Cf. Jn 14:26.