The difference between matters of precept and of counsel is treated of, as shown in the case of the young man in the Gospel, and the difference of the rewards set forth both for counsels and precepts is spoken of.
72. Marriage, then, is honourable, but chastity is more honourable, for “he that gives his virgin in marriage does well, but he that gives her not in marriage does better.” That, then, which is good need not be avoided, but that which is better should be chosen. And so it is not laid upon any, but set before him. And, therefore, the Apostle said well: “Concerning virgins I have no commandment of the Lord, yet I give my counsel.” For a command is issued to those subject, counsel is given to friends. Where there is a commandment, there is a law; where counsel, there is grace. A commandment is given to enforce what is according to nature, a counsel to incite us to follow grace. And, therefore, the Law was given to the Jews, but grace was reserved for the elect. The Law was given that, through fear of punishment, it might recall those who were wandering beyond the limits of nature, to their observance, but grace to incite the elect both by the desire of good things, and also by the promised rewards.
73. You will see the difference between precept and counsel, if you remember the case of him in the Gospel, to whom it is first commanded to do no murder, not to commit adultery, not to bear false witness; for that is a commandment which has a penalty for its transgression. But when he said that he had fulfilled all the commandments of the Law, there is given to him a counsel that he should sell all that he had and follow the Lord, for these things are not imposed as commands, but are offered as counsels. For there are two ways of commanding things, one by way of precept, the other by way of counsel. And so the Lord in one way says: “You shall not kill,” where He gives a commandment; in the other He says: “If you will be perfect, sell all that you have.” He is, then, not bound by a commandment to whom the choice is left.
74. And so they who have fulfilled the commandments are able to say: “We are unprofitable servants, we have done that which was our duty to do.” The virgin does not say this, nor he who sold all his goods, but they rather await the stored-up rewards like the holy Apostle who says: “Behold we have forsaken all and followed You, what shall we have therefore?” He says not, like the unprofitable servant, that he has done that which was his duty to do, but as being profitable to his Master, because he has multiplied the talents entrusted to him by the increase he has gained, having a good conscience, and without anxiety as to his merits he expects the reward of his faith and virtue. And so it is said to him and the others: “You which have followed Me, in the regeneration, when the Son of Man shall sit in the throne of His glory, shall also yourselves sit upon twelve thrones, judging the tribes of Israel.” And to those who had faithfully preserved their talents He promises rewards indeed, though smaller saying: “Because you have been faithful over a few things, I will make you ruler over many things.” Good faith, then, is due, but mercy is in the rewards. He who has kept good faith has deserved that good faith should be kept with him; he who has made good profit, because he has not sought his own benefit, has gained a claim to a heavenly reward.
Source: Concerning Widows (New Advent)