Objection 1. It would seem that no precept should be given about charity. For charity imposes the mode on all acts of virtue, since it is the form of the virtues as stated above (Question 23, Article 8), while the precepts are about the virtues themselves. Now, according to the common saying, the mode is not included in the precept. Therefore no precepts should be given about charity.
Objection 2. Further, charity, which "is poured forth in our hearts by the Holy Ghost" (Romans 5:5), makes us free, since "where the Spirit of the Lord is, there is liberty" (2 Corinthians 3:17). Now the obligation that arises from a precept is opposed to liberty, since it imposes a necessity. Therefore no precept should be given about charity.
Objection 3. Further, charity is the foremost among all the virtues, to which the precepts are directed, as shown above (I-II, 90, 2; I-II, 100, 09). If, therefore, any precepts were given about charity, they should have a place among the chief precepts which are those of the decalogue.
But they have no place there. Therefore no precepts should be given about charity.
On the contrary, Whatever God requires of us is included in a precept. Now God requires that man should love Him, according to Deuteronomy 10:12. Therefore it behooved precepts to be given about the love of charity, which is the love of God.
I answer that, As stated above (16, 1; I-II, 99, 1), a precept implies the notion of something due. Hence a thing is a matter of precept, in so far as it is something due. Now a thing is due in two ways, for its own sake, and for the sake of something else. On every affair, it is the end that is due for its own sake, because it has the character of a good for its own sake: while that which is directed to the end is due for the sake of something else: thus for a physician, it is due for its own sake, that he should heal, while it is due for the sake of something else that he should give a medicine in order to heal. Now the end of the spiritual life is that man be united to God, and this union is effected by charity, while all things pertaining to the spiritual life are ordained to this union, as to their end. Hence the Apostle says (1 Timothy 1:5): "The end of the commandment is charity from a pure heart, and a good conscience, and an unfeigned faith." For all the virtues, about whose acts the precepts are given, are directed either to the freeing of the heart from the whirl of the passions--such are the virtues that regulate the passions--or at least to the possession of a good conscience--such are the virtues that regulate operations--or to the having of a right faith--such are those which pertain to the worship of God: and these three things are required of man that he may love God. For an impure heart is withdrawn from loving God, on account of the passion that inclines it to earthly things; an evil conscience gives man a horror for God's justice, through fear of His punishments; and an untrue faith draws man's affections to an untrue representation of God, and separates him from the truth of God. Now in every genus that which is for its own sake takes precedence of that which is for the sake of another, wherefore the greatest precept is that of charity, as stated in Matthew 22:39.
Reply to Objection 1. As stated above (I-II, 100, 10) when we were treating of the commandments, the mode of love does not come under those precepts which are about the other acts of virtue: for instance, this precept, "Honor thy father and thy mother," does not prescribe that this should be done out of charity. The act of love does, however, fall under special precepts.
Reply to Objection 2. The obligation of a precept is not opposed to liberty, except in one whose mind is averted from that which is prescribed, as may be seen in those who keep the precepts through fear alone. But the precept of love cannot be fulfilled save of one's own will, wherefore it is not opposed to charity.
Reply to Objection 3. All the precepts of the decalogue are directed to the love of God and of our neighbor: and therefore the precepts of charity had not to be enumerated among the precepts of the decalogue, since they are included in all of them.
Source: Summa Theologica (New Advent)