Objection 1. It seems that obedience is the greatest of the virtues. For it is written (1 Samuel 15:22): "Obedience is better than sacrifices." Now the offering of sacrifices belongs to religion, which is the greatest of all moral virtues, as shown above (Question 81, Article 6). Therefore obedience is the greatest of all virtues.
Objection 2. Further, Gregory says (Moral. xxxv) that "obedience is the only virtue that ingrafts virtues in the soul and protects them when ingrafted." Now the cause is greater than the effect. Therefore obedience is greater than all the virtues.
Objection 3. Further, Gregory says (Moral. xxxv) that "evil should never be done out of obedience: yet sometimes for the sake of obedience we should lay aside the good we are doing." Now one does not lay aside a thing except for something better. Therefore obedience, for whose sake the good of other virtues is set aside, is better than other virtues.
On the contrary, obedience deserves praise because it proceeds from charity: for Gregory says (Moral. xxxv) that "obedience should be practiced, not out of servile fear, but from a sense of charity, not through fear of punishment, but through love of justice." Therefore charity is a greater virtue than obedience.
I answer that, Just as sin consists in man contemning God and adhering to mutable things, so the merit of a virtuous act consists in man contemning created goods and adhering to God as his end. Now the end is greater than that which is directed to the end. Therefore if a man contemns created goods in order that he may adhere to God, his virtue derives greater praise from his adhering to God than from his contemning earthly things. And so those, namely the theological, virtues whereby he adheres to God in Himself, are greater than the moral virtues, whereby he holds in contempt some earthly thing in order to adhere to God.
Among the moral virtues, the greater the thing which a man contemns that he may adhere to God, the greater the virtue. Now there are three kinds of human goods that man may contemn for God's sake. The lowest of these are external goods, the goods of the body take the middle place, and the highest are the goods of the soul; and among these the chief, in a way, is the will, in so far as, by his will, man makes use of all other goods. Therefore, properly speaking, the virtue of obedience, whereby we contemn our own will for God's sake, is more praiseworthy than the other moral virtues, which contemn other goods for the sake of God.
Hence Gregory says (Moral. xxxv) that "obedience is rightly preferred to sacrifices, because by sacrifices another's body is slain whereas by obedience we slay our own will." Wherefore even any other acts of virtue are meritorious before God through being performed out of obedience to God's will. For were one to suffer even martyrdom, or to give all one's goods to the poor, unless one directed these things to the fulfilment of the divine will, which pertains directly to obedience, they could not be meritorious: as neither would they be if they were done without charity, which cannot exist apart from obedience. For it is written (1 John 2:4-5): "He who saith that he knoweth God, and keepeth not His commandments, is a liar... but he that keepeth His word, in him in very deed the charity of God is perfected": and this because friends have the same likes and dislikes.
Reply to Objection 1. Obedience proceeds from reverence, which pays worship and honor to a superior, and in this respect it is contained under different virtues, although considered in itself, as regarding the aspect of precept, it is one special virtue. Accordingly, in so far as it proceeds from reverence for a superior, it is contained, in a way, under observance; while in so far as it proceeds from reverence for one's parents, it is contained under piety; and in so far as it proceeds from reverence for God, it comes under religion, and pertains to devotion, which is the principal act of religion. Wherefore from this point of view it is more praiseworthy to obey God than to offer sacrifice, as well as because, "in a sacrifice we slay another's body, whereas by obedience we slay our own will," as Gregory says (Moral. xxxv). As to the special case in which Samuel spoke, it would have been better for Saul to obey God than to offer in sacrifice the fat animals of the Amalekites against the commandment of God.
Reply to Objection 2. All acts of virtue, in so far as they come under a precept, belong to obedience. Wherefore according as acts of virtue act causally or dispositively towards their generation and preservation, obedience is said to ingraft and protect all virtues. And yet it does not follow that obedience takes precedence of all virtues absolutely, for two reasons. First, because though an act of virtue come under a precept, one may nevertheless perform that act of virtue without considering the aspect of precept. Consequently, if there be any virtue, whose object is naturally prior to the precept, that virtue is said to be naturally prior to obedience. Such a virtue is faith, whereby we come to know the sublime nature of divine authority, by reason of which the power to command is competent to God. Secondly, because infusion of grace and virtues may precede, even in point of time, all virtuous acts: and in this way obedience is not prior to all virtues, neither in point of time nor by nature.
Reply to Objection 3. There are two kinds of good. There is that to which we are bound of necessity, for instance to love God, and so forth: and by no means may such a good be set aside on account of obedience. But there is another good to which man is not bound of necessity, and this good we ought sometimes to set aside for the sake of obedience to which we are bound of necessity, since we ought not to do good by falling into sin. Yet as Gregory remarks (Moral. xxxv), "he who forbids his subjects any single good, must needs allow them many others, lest the souls of those who obey perish utterly from starvation, through being deprived of every good." Thus the loss of one good may be compensated by obedience and other goods.
Source: Summa Theologica (New Advent)