Objection 1. It would seem that penance is the first of the virtues. Because, on Matthew 3:2, "Do penance," etc., a gloss says: "The first virtue is to destroy the old man, and hate sin by means of penance."
Objection 2. Further, withdrawal from one extreme seems to precede approach to the other. Now all the other virtues seem to regard approach to a term, because they all direct man to do good; whereas penance seems to direct him to withdraw from evil. Therefore it seems that penance precedes all the other virtues.
Objection 3. Further, before penance, there is sin in the soul. Now no virtue is compatible with sin in the soul. Therefore no virtue precedes penance, which is itself the first of all and opens the door to the others by expelling sin.
On the contrary, Penance results from faith, hope, and charity, as already stated (2,5). Therefore penance is not the first of the virtues.
I answer that, In speaking of the virtues, we do not consider the order of time with regard to the habits, because, since the virtues are connected with one another, as stated in I-II, 65, 1, they all begin at the same time to be in the soul; but one is said to precede the other in the order of nature, which order depends on the order of their acts, in so far as the act of one virtue presupposes the act of another. Accordingly, then, one must say that, even in the order of time, certain praiseworthy acts can precede the act and the habit of penance, e.g. acts of dead faith and hope, and an act of servile fear; while the act and habit of charity are, in point of time, simultaneous with the act and habit of penance, and with the habits of the other virtues. For, as was stated in I-II, 113, 7,8, in the justification of the ungodly, the movement of the free-will towards God, which is an act of faith quickened by charity, and the movement of the free-will towards sin, which is the act of penance, are simultaneous. Yet of these two acts, the former naturally precedes the latter, because the act of the virtue of penance is directed against sin, through love of God; where the first-mentioned act is the reason and cause of the second.
Consequently penance is not simply the first of the virtues, either in the order of time, or in the order of nature, because, in the order of nature, the theological virtues precede it simply. Nevertheless, in a certain respect, it is the first of the other virtues in the order of time, as regards its act, because this act is the first in the justification of the ungodly; whereas in the order of nature, the other virtues seem to precede, as that which is natural precedes that which is accidental; because the other virtues seem to be necessary for man's good, by reason of their very nature, whereas penance is only necessary if something, viz. sin, be presupposed, as stated above (Question 55, Article 2), when we spoke of the relation of the sacrament of penance to the other sacraments aforesaid.
Reply to Objection 1. This gloss is to be taken as meaning that the act of penance is the first in point of time, in comparison with the acts of the other virtues.
Reply to Objection 2. In successive movements withdrawal from one extreme precedes approach to the other, in point of time; and also in the order of nature, if we consider the subject, i.e. the order of the material cause; but if we consider the order of the efficient and final causes, approach to the end is first, for it is this that the efficient cause intends first of all: and it is this order which we consider chiefly in the acts of the soul, as stated in Phys. ii.
Reply to Objection 3. Penance opens the door to the other virtues, because it expels sin by the virtues of faith, hope and charity, which precede it in the order of nature; yet it so opens the door to them that they enter at the same time as it: because, in the justification of the ungodly, at the same time as the free-will is moved towards God and against sin, the sin is pardoned and grace infused, and with grace all the virtues, as stated in I-II, 65, 3,5.
Source: Summa Theologica (New Advent)