As it belongs to one Divine Person to be from another by Origin, as the Son is from the Father, so it also befits the Divinity to be a Gift or Given, and to be possessed by the rational creature; not indeed by its own power, but as, by union with God, it is made partaker of the Divine Word and Love Proceeding, in order that it may freely know God and truly and rightly love Him, Thus the Holy Spirit is called a Gift because we have Him from Another, and He is so called according as He is given to us Personally: for a gift denotes properly that which is given without return; and the conception of gratuitous giving is that of love which wishes good to another. Since, therefore, the Holy Ghost proceeds as Love, He has the nature of the First Gift, by which, according to Augustine, all gratuitous gifts are given and divided among the members of Christ. Thus as the Son is called an Image on account of His likeness to His Principle, so the Holy Spirit proceeding from the Father as Love is properly called a Gift, although the Son is also given.
- text taken from Compendium of the Summa Theologica of Saint Thomas Aquinas, by Bishop Berardus Bongiovanni