St. Thomas Aquinas (1225-1274), the Angelic Doctor, is a towering figure among Catholic theologians and a source of perennial wisdom for understanding the truths of our faith.
The selections that follow come from St. Thomas’ work Summa Contra Gentiles, written to equip Christian preachers and catechists for the task of evangelization.
St. Thomas begins by showing that God’s love has been poured into our hearts through the Holy Spirit. This means the Holy Spirit has truly come to dwell within us— and through the Spirit, the Father and Son also live in us. But if God lives in us, then we too live in God. This indwelling of the Trinity is the foundation for all the various works of the Spirit through us.
In a striking way, St. Thomas then presents the Holy Spirit as the one who brings us into friendship with God. Thomas asks: How do friends act toward one another? Quoting Aristotle — the classical teacher on the meaning of friendship — St. Thomas says friends share with each other what they possess. And so, God the Holy Spirit shares with us, for example, the revelation of the mysteries of God, leading us into all truth. The Spirit also shares with us spiritual gifts, so we can cooperate with divine grace and carry out the mission of Christ.
For St. Thomas, the Holy Spirit is not just a power who works through us: The Spirit comes to dwell in us and makes us friends of God. And as “the friends of God,” we receive from the Holy Spirit all the gifts we need for every good work.
St. Thomas Aquinas on the Holy Spirit: Indwelling, Friendship and Spiritual Gifts 
Since the charity by which we love God is in us by the Holy Spirit, the Holy Spirit Himself must also be in us, so long as the charity is in us. And so the Apostle says: “Do you not know that you are the temple of God, and that the Spirit of God dwells in you?” (1 Cor 3:16). Therefore, since we are made lovers of God by the Holy Spirit, and every beloved is in the lover as such, by the Holy Spirit necessarily the Father and the Son dwell in us also.
Of course, every beloved is in a lover. Therefore, by the Holy Spirit not only is God in us, but we also are in God. Hence, we read in 1 John (4:16, 13): “He who abides in charity abides in God, and God in him,” and “In this we know that we abide in Him and He in us: because He has given us of His Spirit.”
Therefore, since by the Holy Spirit we are established as friends of God, fittingly enough it is by the Holy Spirit that people are said to receive the revelation of the divine mysteries.
Now, it is not only proper to love that one reveal his secrets to a friend by reason of their unity in affection, but the same unity requires that what he has, he have in common with the friend. For, “since a man has a friend as another self,” he must help the friend as he does himself, making his own possessions common with the friend, and so one takes this as the property of friendship “to will and to do the good for a friend.”
Therefore, it is fitting that all the gifts of God are said to be gifts from the Holy Spirit; thus, in 1 Corinthians (12:8, 11): “To one, indeed, by the Spirit is given the word of wisdom, to another, the word of knowledge, according to the same Spirit”; and later on, having mentioned many, it says: “One and the same Spirit works, dividing to every one according as He will.”
Of course, the spiritual gifts are given to us by the Holy Spirit … And thus by the Holy Spirit we are configured to God and through Him we are made ready for good operation. And by the same Spirit the road to beatitude is opened for us.
Thomas Aquinas, Summa Contra Gentiles, Book IV, chap. 21., trans. Charles J. O’Neil (Notre Dame: University of Notre Dame Press, 1975), 122-24, adjusted.
St. Thomas is quoting Aristotle, Nichomachean Ethics, IX.4.