Objection 1. It would seem that the mode of charity falls under the precept of the Divine law. For it is written (Matthew 19:17): "If thou wilt enter into life, keep the commandments": whence it seems to follow that the observance of the commandments suffices for entrance into life. But good works do not suffice for entrance into life, except they be done from charity: for it is written (1 Corinthians 13:3): "If I should distribute all my goods to feed the poor, and if I should deliver my body to be burned, and have not charity, it profiteth me nothing." Therefore the mode of charity is included in the commandment.
Objection 2. Further, the mode of charity consists properly speaking in doing all things for God. But this falls under the precept; for the Apostle says (1 Corinthians 10:31): "Do all to the glory of God." Therefore the mode of charity falls under the precept.
Objection 3. Further, if the mode of charity does not fall under the precept, it follows that one can fulfil the precepts of the law without having charity. Now what can be done without charity can be done without grace, which is always united to charity. Therefore one can fulfil the precepts of the law without grace. But this is the error of Pelagius, as Augustine declares (De Haeres. lxxxviii). Therefore the mode of charity is included in the commandment.
On the contrary, Whoever breaks a commandment sins mortally. If therefore the mode of charity falls under the precept, it follows that whoever acts otherwise than from charity sins mortally. But whoever has not charity, acts otherwise than from charity. Therefore it follows that whoever has not charity, sins mortally in whatever he does, however good this may be in itself: which is absurd.
I answer that, Opinions have been contrary on this question. For some have said absolutely that the mode of charity comes under the precept; and yet that it is possible for one not having charity to fulfil this precept: because he can dispose himself to receive charity from God. Nor (say they) does it follow that a man not having charity sins mortally whenever he does something good of its kind: because it is an affirmative precept that binds one to act from charity, and is binding not for all time, but only for such time as one is in a state of charity. On the other hand, some have said that the mode of charity is altogether outside the precept.
Both these opinions are true up to a certain point. Because the act of charity can be considered in two ways. First, as an act by itself: and thus it falls under the precept of the law which specially prescribes it, viz. "Thou shalt love the thy Lord thy God," and "Thou shalt love thy neighbor." In this sense, the first opinion is true. Because it is not impossible to observe this precept which regards the act of charity; since man can dispose himself to possess charity, and when he possesses it, he can use it. Secondly, the act of charity can be considered as being the mode of the acts of the other virtues, i.e. inasmuch as the acts of the other virtues are ordained to charity, which is "the end of the commandment," as stated in 1 Timothy 1:5: for it has been said above (Question 12, Article 4) that the intention of the end is a formal mode of the act ordained to that end. In this sense the second opinion is true in saying that the mode of charity does not fall under the precept, that is to say that this commandment, "Honor thy father," does not mean that a man must honor his father from charity, but merely that he must honor him. Wherefore he that honors his father, yet has not charity, does not break this precept: although he does break the precept concerning the act of charity, for which reason he deserves to be punished.
Reply to Objection 1. Our Lord did not say, "If thou wilt enter into life, keep one commandment"; but "keep" all "the commandments": among which is included the commandment concerning the love of God and our neighbor.
Reply to Objection 2. The precept of charity contains the injunction that God should be loved from our whole heart, which means that all things would be referred to God. Consequently man cannot fulfil the precept of charity, unless he also refer all things to God. Wherefore he that honors his father and mother, is bound to honor them from charity, not in virtue of the precept, "Honor thy father and mother," but in virtue of the precept, "Thou shalt love the Lord thy God with thy whole heart." And since these are two affirmative precepts, not binding for all times, they can be binding, each one at a different time: so that it may happen that a man fulfils the precept of honoring his father and mother, without at the same time breaking the precept concerning the omission of the mode of charity.
Reply to Objection 3. Man cannot fulfil all the precepts of the law, unless he fulfil the precept of charity, which is impossible without charity. Consequently it is not possible, as Pelagius maintained, for man to fulfil the law without grace.
Source: Summa Theologica (New Advent)