It befits the Divine Persons to be Sent, but not so as that the One Sent implies any inferiority to the Sender, or change of place (as if anything should begin to be where formerly it was not); it denotes, rather, a Procession of Origin from the Sender, and a kind of new mode of existence. Thus the Son, sent into the world by the Father, began to be in the world by assuming human nature, and yet was already in the world, as we read in the first chapter of Saint John.
The words Mission and Gift are used of the Divinity only temporally; Generation and Spiration only eternally; while Procession or Outgoing is used both eternally and temporally. For the Son proceeds eternally as He is in God, and temporally, according to Visible Mission as He is also Man, or in man, by Invisible Mission.
The Divine Persons are also said to be sent as They exist in some one in a new manner, and given, according as They are received by any one as a Free Gift; for although God is in everything by Essence, Power and Presence, He exists nevertheless in some special manner in rational creatures. In them God is said to be, as that which is known is in him who knows, or as the loved is in the lover; for the creature itself attains to God by knowing and loving Him through the Divine Operation; so that God is said to dwell in us as in His temple. Therefore we say that the Divine Persons are Sent and Proceed temporally, after the mode of grace, which is given to make us pleasing to God, and which we have the power freely to enjoy.
But although the Father communicates Himself to creatures it does not belong to Him to be sent; for Mission in the Divinity implies, by its nature, Procession from Another and Origin. It does not, therefore, belong to the Father, but only to the Son and to the Holy Ghost, to Whom it belongs both to dwell in us and to be from Another. Indwelling by grace belongs to the Father also, but as He is not from Another, so neither is He said to be Sent.
An Invisible Mission is made to all who participate in grace. For there are two things to be considered: the indwelling of grace and the renovation by grace. Thus the Divine Persons are sent, according to the capacity of those who, by grace, know and understand Them; while, to the Blessed, this Invisible Mission is made the principle of their Beatitude. Afterwards, indeed, another mission may be made, not for the increase of grace, but according as some mystery is further revealed to them; and this will continue until the Day of Judgment.
As it is natural to man to be led by the visible to the invisible, it was necessary that the invisible things of God should be manifested by those which are visible; and as God manifested Himself, and the Eternal Procession of the Divine Persons, in a manner, by certain indications, it was suitable that the Invisible Mission should also be manifested by some visible sign. Thus the Son was sent visibly in Unity of Person, as the Author of sanctification, and the Holy Spirit appeared visibly in certain creatures as a Sign of sanctification, which sign lasted as long as the occasion demanded.
The Divine Persons are said to be Sent according as They Proceed One from Another eternally, if by the Sender we understand the Principle of the Person sent; but if we understand the Principle of the effect which forms the object of the Mission, the Divine Person sent is sent by the whole Trinity.
- text taken from Compendium of the Summa Theologica of Saint Thomas Aquinas, by Bishop Berardus Bongiovanni