Objection 1. It would seem that the habitual grace did not follow after the union. For nothing follows itself. But this habitual grace seems to be the same as the grace of union; for Augustine says (De Praedest. Sanct. xv): "Every man becomes a Christian from the beginning of his belief, by the same grace whereby this Man from His beginning became Christ"; and of these two the first pertains to habitual grace and the second to the grace of union. Therefore it would seem that habitual grace did not follow upon the union.
Objection 2. Further, disposition precedes perfection, if not in time, at least in thought. But the habitual grace seems to be a disposition in human nature for the personal union. Therefore it seems that the habitual grace did not follow but rather preceded the union.
Objection 3. Further, the common precedes the proper. But habitual grace is common to Christ and other men; and the grace of union is proper to Christ. Therefore habitual grace is prior in thought to the union. Therefore it does not follow it.
On the contrary, It is written (Isaiah 42:1): "Behold my servant, I will uphold Him... "and farther on: "I have given My Spirit upon Him"; and this pertains to the gift of habitual grace. Hence it remains that the assumption of human nature to the unity of the Person preceded the habitual grace of Christ.
I answer that, The union of the human nature with the Divine Person, which, as we have said above (2, 10; 6, 6), is the grace of union, precedes the habitual grace of Christ, not in order of time, but by nature and in thought; and this for a triple reason: First, with reference to the order of the principles of both. For the principle of the union is the Person of the Son assuming human nature, Who is said to be sent into the world, inasmuch as He assumed human nature; but the principle of habitual grace, which is given with charity, is the Holy Ghost, Who is said to be sent inasmuch as He dwells in the mind by charity. Now the mission of the Son is prior, in the order of nature, to the mission of the Holy Ghost, even as in the order of nature the Holy Ghost proceeds from the Son, and love from wisdom. Hence the personal union, according to which the mission of the Son took place, is prior in the order of nature to habitual grace, according to which the mission of the Holy Ghost takes place. Secondly, the reason of this order may be taken from the relation of grace to its cause. For grace is caused in man by the presence of the Godhead, as light in the air by the presence of the sun. Hence it is written (Ezekiel 43:2): "The glory of the God of Israel came in by the way of the east... and the earth shone with His majesty." But the presence of God in Christ is by the union of human nature with the Divine Person. Hence the habitual grace of Christ is understood to follow this union, as light follows the sun. Thirdly, the reason of this union can be taken from the end of grace, since it is ordained to acting rightly, and action belongs to the suppositum and the individual. Hence action and, in consequence, grace ordaining thereto, presuppose the hypostasis which operates. Now the hypostasis did not exist in the human nature before the union, as is clear from 4, 2]. Therefore the grace of union precedes, in thought, habitual grace.
Reply to Objection 1. Augustine here means by grace the gratuitous will of God, bestowing benefits gratis; and hence every man is said to be made a Christian by the same grace whereby a Man became Christ, since both take place by the gratuitous will of God without merits.
Reply to Objection 2. As disposition in the order of generation precedes the perfection to which it disposes, in such things as are gradually perfected; so it naturally follows the perfection which one has already obtained; as heat, which was a disposition to the form of fire, is an effect flowing from the form of already existing fire. Now the human nature in Christ is united to the Person of the Word from the beginning without succession. Hence habitual grace is not understood to have preceded the union, but to have followed it; as a natural property. Hence, as Augustine says (Enchiridion xl): "Grace is in a manner natural to the Man Christ."
Reply to Objection 3. The common precedes the proper, when both are of the same genus; but when they are of divers genera, there is nothing to prevent the proper being prior to the common. Now the grace of union is not in the same genus as habitual grace; but is above all genera even as the Divine Person Himself. Hence there is nothing to prevent this proper from being before the common since it does not result from something being added to the common, but is rather the principle and source of that which is common.
Source: Summa Theologica (New Advent)