Objection 1. It would seem that it is not fitting for the Son to be sent invisibly. For invisible mission of the divine person is according to the gift of grace. But all gifts of grace belong to the Holy Ghost, according to 1 Corinthians 12:11: "One and the same Spirit worketh all things." Therefore only the Holy Ghost is sent invisibly.
Objection 2. Further, the mission of the divine person is according to sanctifying grace. But the gifts belonging to the perfection of the intellect are not gifts of sanctifying grace, since they can be held without the gift of charity, according to 1 Corinthians 13:2: "If I should have prophecy, and should know all mysteries, and all knowledge, and if I should have all faith so that I could move mountains, and have not charity, I am nothing." Therefore, since the Son proceeds as the word of the intellect, it seems unfitting for Him to be sent invisibly.
Objection 3. Further, the mission of the divine person is a procession, as expounded above (1,4). But the procession of the Son and of the Holy Ghost differ from each other. Therefore they are distinct missions if both are sent; and then one of them would be superfluous, since one would suffice for the creature's sanctification.
On the contrary, It is said of divine Wisdom (Wisdom 9:10): "Send her from heaven to Thy Saints, and from the seat of Thy greatness."
I answer that, The whole Trinity dwells in the mind by sanctifying grace, according to John 14:23: "We will come to him, and will make Our abode with him." But that a divine person be sent to anyone by invisible grace signifies both that this person dwells in a new way within him and that He has His origin from another. Hence, since both to the Son and to the Holy Ghost it belongs to dwell in the soul by grace, and to be from another, it therefore belongs to both of them to be invisibly sent. As to the Father, though He dwells in us by grace, still it does not belong to Him to be from another, and consequently He is not sent.
Reply to Objection 1. Although all the gifts, considered as such, are attributed to the Holy Ghost, forasmuch as He is by His nature the first Gift, since He is Love, as stated above (Question 38, Article 1), some gifts nevertheless, by reason of their own particular nature, are appropriated in a certain way to the Son, those, namely, which belong to the intellect, and in respect of which we speak of the mission of the Son. Hence Augustine says (De Trin. iv, 20) that "The Son is sent to anyone invisibly, whenever He is known and perceived by anyone."
Reply to Objection 2. The soul is made like to God by grace. Hence for a divine person to be sent to anyone by grace, there must needs be a likening of the soul to the divine person Who is sent, by some gift of grace. Because the Holy Ghost is Love, the soul is assimilated to the Holy Ghost by the gift of charity: hence the mission of the Holy Ghost is according to the mode of charity. Whereas the Son is the Word, not any sort of word, but one Who breathes forth Love. Hence Augustine says (De Trin. ix 10): "The Word we speak of is knowledge with love." Thus the Son is sent not in accordance with every and any kind of intellectual perfection, but according to the intellectual illumination, which breaks forth into the affection of love, as is said (John 6:45): "Everyone that hath heard from the Father and hath learned, cometh to Me," and (Psalm 38:4): "In my meditation a fire shall flame forth." Thus Augustine plainly says (De Trin. iv, 20): "The Son is sent, whenever He is known and perceived by anyone." Now perception implies a certain experimental knowledge; and this is properly called wisdom [sapientia], as it were a sweet knowledge [sapida scientia], according to Sirach 6:23: "The wisdom of doctrine is according to her name."
Reply to Objection 3. Since mission implies the origin of the person Who is sent, and His indwelling by grace, as above explained (1), if we speak of mission according to origin, in this sense the Son's mission is distinguished from the mission of the Holy Ghost, as generation is distinguished from procession. If we consider mission as regards the effect of grace, in this sense the two missions are united in the root which is grace, but are distinguished in the effects of grace, which consist in the illumination of the intellect and the kindling of the affection. Thus it is manifest that one mission cannot be without the other, because neither takes place without sanctifying grace, nor is one person separated from the other.
Source: Summa Theologica (New Advent)