Encyclical of Pope John Paul II "Lord and Giver of Life," 18 May 1986.
From section 10
In his intimate life, God "is love," the essential love shared by the three divine Persons: personal love is the Holy Spirit as the Spirit of the Father and the Son. Therefore he " searches even the depths of God ", as uncreated Love-Gift. It can be said that in the Holy Spirit the intimate life of the Triune God becomes totally gift, an exchange of mutual love between the divine Persons, and that through the Holy Spirit God exists in the mode of gift. It is the Holy Spirit who is the personal expression of this self-giving, of this being-love. He is Person-Love. He is Person-Gift. Here we have an inexhaustible treasure of the reality and an inexpressible deepening of the concept of person in God, which only divine Revelation makes known to us.
At the same time, the Holy Spirit, being consubstantial with the Father and the Son in divinity, is love and uncreated gift from which derives as from its source (Fons vivus) all giving of gifts vis-a-vis creatures (created gift): the gift of existence to all things through creation; the gift of grace to human beings through the whole economy of salvation. As the Apostle Paul writes: "God's love has been poured into our hearts through the Holy Spirit which has been given to us". The salvific self-giving of God in the Holy Spirit
Excerpted from John Paul II's encyclical letter "Lord and Giver of Life," Dominum et Vivificantem, 18 May 1986.