The Holy Spirit makes present the mystery of Christ
The Holy Spirit's transforming power in the liturgy hastens the coming of the kingdom and the consummation of the mystery of salvation. While we wait in hope he causes us really to anticipate the fullness of communion with the Holy Trinity. Sent by the Father who hears the epiclesis of the Church, the Spirit gives life to those who accept him and is, even now, the "guarantee" of their inheritance.
Source: Catechism of the Catholic Church (Vatican.va)