CIC 243 Before his Passover, Jesus announced the sending of “another Paraclete” (Advocate), the Holy Spirit. At work since creation, having previously “spoken through the prophets”, the Spirit will now be with and in the disciples, to teach them and guide them “into all the truth”.1 The Holy Spirit is thus revealed as another divine person with Jesus and the Father.

CIC 687 “No one comprehends the thoughts of God except the Spirit of God.”2 Now God’s Spirit, who reveals God, makes known to us Christ, his Word, his living Utterance, but the Spirit does not speak of himself. The Spirit who “has spoken through the prophets” makes us hear the Father’s Word, but we do not hear the Spirit himself. We know him only in the movement by which he reveals the Word to us and disposes us to welcome him in faith. The Spirit of truth who “unveils” Christ to us “will not speak on his own.”3 Such properly divine self-effacement explains why “the world cannot receive [him], because it neither sees him nor knows him,” while those who believe in Christ know the Spirit because he dwells with them.4

CIC 729 Only when the hour has arrived for his glorification does Jesus promise the coming of the Holy Spirit, since his Death and Resurrection will fulfill the promise made to the fathers.5 The Spirit of truth, the other Paraclete, will be given by the Father in answer to Jesus’ prayer; he will be sent by the Father in Jesus’ name; and Jesus will send him from the Father’s side, since he comes from the Father. The Holy Spirit will come and we shall know him; he will be with us for ever; he will remain with us. The Spirit will teach us everything, remind us of all that Christ said to us and bear witness to him. The Holy Spirit will lead us into all truth and will glorify Christ. He will prove the world wrong about sin, righteousness, and judgment.

CIC 788 When his visible presence was taken from them, Jesus did not leave his disciples orphans. He promised to remain with them until the end of time; he sent them his Spirit.6 As a result communion with Jesus has become, in a way, more intense: “By communicating his Spirit, Christ mystically constitutes as his body those brothers of his who are called together from every nation.”7

CIC 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.”8 This new dimension of prayer and of its circumstances is displayed throughout the farewell discourse.9 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.”10

CIC 2671 The traditional form of petition to the Holy Spirit is to invoke the Father through Christ our Lord to give us the Consoler Spirit.11 Jesus insists on this petition to be made in his name at the very moment when he promises the gift of the Spirit of Truth.12 But the simplest and most direct prayer is also traditional, “Come, Holy Spirit,” and every liturgical tradition has developed it in antiphons and hymns.
Come, Holy Spirit, fill the hearts of your faithful and enkindle in them the fire of your love.13
Heavenly King, Consoler Spirit, Spirit of Truth, present everywhere and filling all things, treasure of all good and source of all life, come dwell in us, cleanse and save us, you who are All Good.14

1 Cf. Gen 1:2; Nicene Creed (DS 150); Jn 14:17, 26; 16:13.
2 1 Cor 2:11.
3 Jn 16:13.
4 Jn 14:17.
5 Cf. Jn 14:16-17, 26; 15:26; 16:7-15; 17:26.
6 Cf. Jn 14:18; 20:22; Mt 28:20; Acts 2:33.
7 LG 7.
8 Jn 14:16-17.
9 Cf. Jn 14:23-26; 15:7, 16; 16:13-15; 16:23-27.
10 Jn 16:24.
11 Cf. Lk 11:13.
12 Cf. Jn 14:17; 15:26; 16:13.
13 Roman Missal, Pentecost Sequence.
14 Byzantine Liturgy, Pentecost Vespers, Troparion.