Objection 1. It would seem that the New Law made insufficient ordinations about external acts. Because faith that worketh through charity seems chiefly to belong to the New Law, according to Galatians 5:6: "In Christ Jesus neither circumcision availeth anything, nor uncircumcision: but faith that worketh through charity." But the New Law declared explicitly certain points of faith which were not set forth explicitly in the Old Law; for instance, belief in the Trinity. Therefore it should also have added certain outward moral deeds, which were not fixed in the Old Law.
Objection 2. Further, in the Old Law not only were sacraments instituted, but also certain sacred things, as stated above (101, 4; 102, 4). But in the New Law, although certain sacraments are instituted by Our Lord; for instance, pertaining either to the sanctification of a temple or of the vessels, or to the celebration of some particular feast. Therefore the New Law made insufficient ordinations about external matters.
Objection 3. Further, in the Old Law, just as there were certain observances pertaining to God's ministers, so also were there certain observances pertaining to the people: as was stated above when we were treating of the ceremonial of the Old Law (101, 4; 102, 6). Now in the New Law certain observances seem to have been prescribed to the ministers of God; as may be gathered from Matthew 10:9: "Do not possess gold, nor silver, nor money in your purses," nor other things which are mentioned here and Luke 9 and 10. Therefore certain observances pertaining to the faithful should also have been instituted in the New Law.
Objection 4. Further, in the Old Law, besides moral and ceremonial precepts, there were certain judicial precepts. But in the New Law there are no judicial precepts. Therefore the New Law made insufficient ordinations about external works.
On the contrary, Our Lord said (Matthew 7:24): "Every one... that heareth these My words, and doth them, shall be likened to a wise man that built his house upon a rock." But a wise builder leaves out nothing that is necessary to the building. Therefore Christ's words contain all things necessary for man's salvation.
I answer that, as stated above (Article 1), the New Law had to make such prescriptions or prohibitions alone as are essential for the reception or right use of grace. And since we cannot of ourselves obtain grace, but through Christ alone, hence Christ of Himself instituted the sacraments whereby we obtain grace: viz. Baptism, Eucharist, Orders of the ministers of the New Law, by the institution of the apostles and seventy-two disciples, Penance, and indissoluble Matrimony. He promised Confirmation through the sending of the Holy Ghost: and we read that by His institution the apostles healed the sick by anointing them with oil (Mark 6:13). These are the sacraments of the New Law.
The right use of grace is by means of works of charity. These, in so far as they are essential to virtue, pertain to the moral precepts, which also formed part of the Old Law. Hence, in this respect, the New Law had nothing to add as regards external action. The determination of these works in their relation to the divine worship, belongs to the ceremonial precepts of the Law; and, in relation to our neighbor, to the judicial precepts, as stated above (Question 99, Article 4). And therefore, since these determinations are not in themselves necessarily connected with inward grace wherein the Law consists, they do not come under a precept of the New Law, but are left to the decision of man; some relating to inferiors--as when a precept is given to an individual; others, relating to superiors, temporal or spiritual, referring, namely, to the common good.
Accordingly the New Law had no other external works to determine, by prescribing or forbidding, except the sacraments, and those moral precepts which have a necessary connection with virtue, for instance, that one must not kill, or steal, and so forth.
Reply to Objection 1. Matters of faith are above human reason, and so we cannot attain to them except through grace. Consequently, when grace came to be bestowed more abundantly, the result was an increase in the number of explicit points of faith. On the other hand, it is through human reason that we are directed to works of virtue, for it is the rule of human action, as stated above (19, 3; 63, 2). Wherefore in such matters as these there was no need for any precepts to be given besides the moral precepts of the Law, which proceed from the dictate of reason.
Reply to Objection 2. In the sacraments of the New Law grace is bestowed, which cannot be received except through Christ: consequently they had to be instituted by Him. But in the sacred things no grace is given: for instance, in the consecration of a temple, an altar or the like, or, again, in the celebration of feasts. Wherefore Our Lord left the institution of such things to the discretion of the faithful, since they have not of themselves any necessary connection with inward grace.
Reply to Objection 3. Our Lord gave the apostles those precepts not as ceremonial observances, but as moral statutes: and they can be understood in two ways. First, following Augustine (De Consensu Evang. 30), as being not commands but permissions. For He permitted them to set forth to preach without scrip or stick, and so on, since they were empowered to accept their livelihood from those to whom they preached: wherefore He goes on to say: "For the laborer is worthy of his hire." Nor is it a sin, but a work of supererogation for a preacher to take means of livelihood with him, without accepting supplies from those to whom he preaches; as Paul did (1 Corinthians 9:4, seqq.).
Secondly, according to the explanation of other holy men, they may be considered as temporal commands laid upon the apostles for the time during which they were sent to preach in Judea before Christ's Passion. For the disciples, being yet as little children under Christ's care, needed to receive some special commands from Christ, such as all subjects receive from their superiors: and especially so, since they were to be accustomed little by little to renounce the care of temporalities, so as to become fitted for the preaching of the Gospel throughout the whole world. Nor must we wonder if He established certain fixed modes of life, as long as the state of the Old Law endured and the people had not as yet achieved the perfect liberty of the Spirit. These statutes He abolished shortly before His Passion, as though the disciples had by their means become sufficiently practiced. Hence He said (Luke 22:35-36) "When I sent you without purse and scrip and shoes, did you want anything? But they said: Nothing. Then said He unto them: But now, he that hath a purse, let him take it, and likewise a scrip." Because the time of perfect liberty was already at hand, when they would be left entirely to their own judgment in matters not necessarily connected with virtue.
Reply to Objection 4. Judicial precepts also, are not essential to virtue in respect of any particular determination, but only in regard to the common notion of justice. Consequently Our Lord left the judicial precepts to the discretion of those who were to have spiritual or temporal charge of others. But as regards the judicial precepts of the Old Law, some of them He explained, because they were misunderstood by the Pharisees, as we shall state later on (3, ad 2).
Source: Summa Theologica (New Advent)