Objection 1. It would seem that the New Law should not prescribe or prohibit any external acts. For the New Law is the Gospel of the kingdom, according to Matthew 24:14: "This Gospel of the kingdom shall be preached in the whole world." But the kingdom of God consists not in exterior, but only in interior acts, according to Luke 17:21: "The kingdom of God is within you"; and Romans 14:17: "The kingdom of God is not meat and drink; but justice and peace and joy in the Holy Ghost." Therefore the New Law should not prescribe or forbid any external acts.
Objection 2. Further, the New Law is "the law of the Spirit" (Romans 8:2). But "where the Spirit of the Lord is, there is liberty" (2 Corinthians 3:17). Now there is no liberty when man is bound to do or avoid certain external acts. Therefore the New Law does not prescribe or forbid any external acts.
Objection 3. Further, all external acts are understood as referable to the hand, just as interior acts belong to the mind. But this is assigned as the difference between the New and Old Laws that the "Old Law restrains the hand, whereas the New Law curbs the will" [Peter Lombard, Sent. iii, D, 40. Therefore the New Law should not contain prohibitions and commands about exterior deeds, but only about interior acts.
On the contrary, Through the New Law, men are made "children of light": wherefore it is written (John 12:36): "Believe in the light that you may be the children of light." Now it is becoming that children of the light should do deeds of light and cast aside deeds of darkness, according to Ephesians 5:8: "You were heretofore darkness, but now light in the Lord. Walk... as children of the light." Therefore the New Law had to forbid certain external acts and prescribe others.
I answer that, As stated above (106, 1,2), the New Law consists chiefly in the grace of the Holy Ghost, which is shown forth by faith that worketh through love. Now men become receivers of this grace through God's Son made man, Whose humanity grace filled first, and thence flowed forth to us. Hence it is written (John 1:14): "The Word was made flesh," and afterwards: "full of grace and truth"; and further on: "Of His fulness we all have received, and grace for grace." Hence it is added that "grace and truth came by Jesus Christ." Consequently it was becoming that the grace flows from the incarnate Word should be given to us by means of certain external sensible objects; and that from this inward grace, whereby the flesh is subjected to the Spirit, certain external works should ensue.
Accordingly external acts may have a twofold connection with grace. In the first place, as leading in some way to grace. Such are the sacramental acts which are instituted in the New Law, e.g. Baptism, the Eucharist, and the like.
In the second place there are those external acts which ensue from the promptings of grace: and herein we must observe a difference. For there are some which are necessarily in keeping with, or in opposition to inward grace consisting in faith that worketh through love. Such external works are prescribed or forbidden in the New Law; thus confession of faith is prescribed, and denial of faith is forbidden; for it is written (Matthew 10:32-33) "(Every one) that shall confess Me before men, I will also confess him before My Father... But he that shall deny Me before men, I will also deny him before My Father." On the other hand, there are works which are not necessarily opposed to, or in keeping with faith that worketh through love. Such works are not prescribed or forbidden in the New Law, by virtue of its primitive institution; but have been left by the Lawgiver, i.e. Christ, to the discretion of each individual. And so to each one it is free to decide what he should do or avoid; and to each superior, to direct his subjects in such matters as regards what they must do or avoid. Wherefore also in this respect the Gospel is called the "law of liberty" [Cf. Reply to Objection 2]: since the Old Law decided many points and left few to man to decide as he chose.
Reply to Objection 1. The kingdom of God consists chiefly in internal acts: but as a consequence all things that are essential to internal acts belong also to the kingdom of God. Thus if the kingdom of God is internal righteousness, peace, and spiritual joy, all external acts that are incompatible with righteousness, peace, and spiritual joy, are in opposition to the kingdom of God; and consequently should be forbidden in the Gospel of the kingdom. On the other hand, those things that are indifferent as regards the aforesaid, for instance, to eat of this or that food, are not part of the kingdom of God; wherefore the Apostle says before the words quoted: "The kingdom of God is not meat and drink."
Reply to Objection 2. According to the Philosopher (Metaph. i, 2), what is "free is cause of itself." Therefore he acts freely, who acts of his own accord. Now man does of his own accord that which he does from a habit that is suitable to his nature: since a habit inclines one as a second nature. If, however, a habit be in opposition to nature, man would not act according to his nature, but according to some corruption affecting that nature. Since then the grace of the Holy Ghost is like an interior habit bestowed on us and inclining us to act aright, it makes us do freely those things that are becoming to grace, and shun what is opposed to it.
Accordingly the New Law is called the law of liberty in two respects. First, because it does not bind us to do or avoid certain things, except such as are of themselves necessary or opposed to salvation, and come under the prescription or prohibition of the law. Secondly, because it also makes us comply freely with these precepts and prohibitions, inasmuch as we do so through the promptings of grace. It is for these two reasons that the New Law is called "the law of perfect liberty" (James 1:25).
Reply to Objection 3. The New Law, by restraining the mind from inordinate movements, must needs also restrain the hand from inordinate acts, which ensue from inward movements.
Source: Summa Theologica (New Advent)