Objection 1. It would seem that the person proceeding is not co-eternal with His principle, as the Son with the Father. For Arius gives twelve modes of generation. The first mode is like the issue of a line from a point; wherein is wanting equality of simplicity. The second is like the emission of rays from the sun; wherein is absent equality of nature. The third is like the mark or impression made by a seal; wherein is wanting consubstantiality and executive power. The fourth is the infusion of a good will from God; wherein also consubstantiality is wanting. The fifth is the emanation of an accident from its subject; but the accident has no subsistence. The sixth is the abstraction of a species from matter, as sense receives the species from the sensible object; wherein is wanting equality of spiritual simplicity. The seventh is the exciting of the will by knowledge, which excitation is merely temporal. The eighth is transformation, as an image is made of brass; which transformation is material. The ninth is motion from a mover; and here again we have effect and cause. The tenth is the taking of species from genera; but this mode has no place in God, for the Father is not predicated of the Son as the genus of a species. The eleventh is the realization of an idea [ideatio], as an external coffer arises from the one in the mind. The twelfth is birth, as a man is begotten of his father; which implies priority and posteriority of time. Thus it is clear that equality of nature or of time is absent in every mode whereby one thing is from another. So if the Son is from the Father, we must say that He is less than the Father, or later than the Father, or both.
Objection 2. Further, everything that comes from another has a principle. But nothing eternal has a principle. Therefore the Son is not eternal; nor is the Holy Ghost.
Objection 3. Further, everything which is corrupted ceases to be. Hence everything generated begins to be; for the end of generation is existence. But the Son is generated by the Father. Therefore He begins to exist, and is not co-eternal with the Father.
Objection 4. Further, if the Son be begotten by the Father, either He is always being begotten, or there is some moment in which He is begotten. If He is always being begotten, since, during the process of generation, a thing must be imperfect, as appears in successive things, which are always in process of becoming, as time and motion, it follows that the Son must be always imperfect, which cannot be admitted. Thus there is a moment to be assigned for the begetting of the Son, and before that moment the Son did not exist.
On the contrary, Athanasius declares that "all the three persons are co-eternal with each other."
I answer that, We must say that the Son is co-eternal with the Father. In proof of which we must consider that for a thing which proceeds from a principle to be posterior to its principle may be due to two reasons: one on the part of the agent, and the other on the part of the action. On the part of the agent this happens differently as regards free agents and natural agents. In free agents, on account of the choice of time; for as a free agent can choose the form it gives to the effect, as stated above (Question 41, Article 2), so it can choose the time in which to produce its effect. In natural agents, however, the same happens from the agent not having its perfection of natural power from the very first, but obtaining it after a certain time; as, for instance, a man is not able to generate from the very first. Considered on the part of action, anything derived from a principle cannot exist simultaneously with its principle when the action is successive. So, given that an agent, as soon as it exists, begins to act thus, the effect would not exist in the same instant, but in the instant of the action's termination. Now it is manifest, according to what has been said (41, 2), that the Father does not beget the Son by will, but by nature; and also that the Father's nature was perfect from eternity; and again that the action whereby the Father produces the Son is not successive, because thus the Son would be successively generated, and this generation would be material, and accompanied with movement; which is quite impossible. Therefore we conclude that the Son existed whensoever the Father existed and thus the Son is co-eternal with the Father, and likewise the Holy Ghost is co-eternal with both.
Reply to Objection 1. As Augustine says (De Verbis Domini, Serm. 38), no mode of the procession of any creature perfectly represents the divine generation. Hence we need to gather a likeness of it from many of these modes, so that what is wanting in one may be somewhat supplied from another; and thus it is declared in the council of Ephesus: "Let Splendor tell thee that the co-eternal Son existed always with the Father; let the Word announce the impassibility of His birth; let the name Son insinuate His consubstantiality." Yet, above them all the procession of the word from the intellect represents it more exactly; the intellectual word not being posterior to its source except in an intellect passing from potentiality to act; and this cannot be said of God.
Reply to Objection 2. Eternity excludes the principle of duration, but not the principle of origin.
Reply to Objection 3. Every corruption is a change; and so all that corrupts begins not to exist and ceases to be. The divine generation, however, is not changed, as stated above (Question 27, Article 2). Hence the Son is ever being begotten, and the Father is always begetting.
Reply to Objection 4. In time there is something indivisible--namely, the instant; and there is something else which endures--namely, time. But in eternity the indivisible "now" stands ever still, as we have said above (10, 2 ad 1, 4 ad 2). But the generation of the Son is not in the "now" of time, or in time, but in eternity. And so to express the presentiality and permanence of eternity, we can say that "He is ever being born," as Origen said (Hom. in Joan. i). But as Gregory [Moral. xxix, 21] and Augustine [Super Ps. 2:7 said, it is better to say "ever born," so that "ever" may denote the permanence of eternity, and "born" the perfection of the only Begotten. Thus, therefore, neither is the Son imperfect, nor "was there a time when He was not," as Arius said.
Source: Summa Theologica (New Advent)