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 Eph. 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.That seems pretty obvious: Christ would have us be children of light; St. Paul commands us to walk as children of light; ergo there must be some rule by which we know what it means to walk as children of light.
...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."
This is not to say that the New Law is as extensive as the old one.
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."Nevertheless, we cannot expect to see the Lord's face if we ignore what he commands: "If you love me, keep my commandments" (John 14:15). It is not enough simply to believe.