Objection 1. It would seem that mission can be eternal. For Gregory says (Hom. xxvi, in Ev.), "The Son is sent as He is begotten." But the Son's generation is eternal. Therefore mission is eternal.
Objection 2. Further, a thing is changed if it becomes something temporally. But a divine person is not changed. Therefore the mission of a divine person is not temporal, but eternal.
Objection 3. Further, mission implies procession. But the procession of the divine persons is eternal. Therefore mission is also eternal.
On the contrary, It is said (Galatians 4:4): "When the fullness of the time was come, God sent His Son."
I answer that, A certain difference is to be observed in all the words that express the origin of the divine persons. For some express only relation to the principle, as "procession" and "going forth." Others express the term of procession together with the relation to the principle. Of these some express the eternal term, as "generation" and "spiration"; for generation is the procession of the divine person into the divine nature, and passive spiration is the procession of the subsisting love. Others express the temporal term with the relation to the principle, as "mission" and "giving." For a thing is sent that it may be in something else, and is given that it may be possessed; but that a divine person be possessed by any creature, or exist in it in a new mode, is temporal.
Hence "mission" and "giving" have only a temporal significance in God; but "generation" and "spiration" are exclusively eternal; whereas "procession" and "giving," in God, have both an eternal and a temporal signification: for the Son may proceed eternally as God; but temporally, by becoming man, according to His visible mission, or likewise by dwelling in man according to His invisible mission.
Reply to Objection 1. Gregory speaks of the temporal generation of the Son, not from the Father, but from His mother; or it may be taken to mean that He could be sent because eternally begotten.
Reply to Objection 2. That a divine person may newly exist in anyone, or be possessed by anyone in time, does not come from change of the divine person, but from change in the creature; as God Himself is called Lord temporally by change of the creature.
Reply to Objection 3. Mission signifies not only procession from the principle, but also determines the temporal term of the procession. Hence mission is only temporal. Or we may say that it includes the eternal procession, with the addition of a temporal effect. For the relation of a divine person to His principle must be eternal. Hence the procession may be called a twin procession, eternal and temporal, not that there is a double relation to the principle, but a double term, temporal and eternal.
Source: Summa Theologica (New Advent)