9 Here therefore these men too evil, while they essay to make void the Law, force us to approve these Scriptures. For they mark what is said, that they who are under the Law are in bondage, and they keep flying above the rest that last saying, “You are made empty of Christ, as many of you as are justified in the Law; you have fallen from Grace.” We grant that all these things are true, and we say that the Law is not necessary, save for them unto whom bondage is yet profitable: and that the Law was on this account profitably enacted, in that men, who could not be recalled from sins by reason, needed to be restrained by such a Law, that is to say, by the threats and terrors of those punishments which can be seen by fools: from which when the Grace of Christ sets us free, it condemns not that Law, but invites us at length to yield obedience to its love, not to be slaves to the fear of the Law.
Itself is Grace, that is free gift, which they understand not to have come to them from God, who still desire to be under the bonds of the Law. Whom Paul deservedly rebukes as unbelievers, because they do not believe that now through our Lord Jesus they have been set free from that bondage, under which they were placed for a certain time by the most just appointment of God. Hence is that saying of the same Apostle, “For the Law was our schoolmaster in Christ.” He therefore gave to men a schoolmaster to fear, Who after gave a Master to love.
And yet in these precepts and commands of the Law, which now it is not allowed Christians to use, such as either the Sabbath, or Circumcision, or Sacrifices, and if there be any thing of this kind, so great mysteries are contained, as that every pious person may understand, there is nothing more deadly than that whatever is there be understood to the letter, that is, to the word: and nothing more healthful than that it be unveiled in the Spirit. Hence it is: “The letter kills, but the Spirit quickens.” Hence it is, “That same veil remains in the reading of the Old Testament, which veil is not taken away; since it is made void in Christ.” For there is made void in Christ, not the Old Testament, but its veil: that so through Christ that may be understood, and, as it were, laid bare, which without Christ is obscure and covered.
Forasmuch as the same Apostle straightway adds, “But when you shall have passed over to Christ, the veil shall be taken away.” For he says not, the Law shall be taken away, or, the Old Testament. Not therefore through the Grace of the Lord, as though useless things were there hidden, have they been taken away; but rather the covering whereby useful things were covered. In this manner all they are dealt with, who earnestly and piously, not disorderly and shamelessly, seek the sense of those Scriptures, and they are carefully shown both the order of events, and the causes of deeds and words, and so great agreement of the Old Testament with the New, that there is left no jot that agrees not; and so great secrets of figures, that all the things that are drawn forth by interpretation force them to confess that they are wretched, who will to condemn these before they learn them.
Source: On the Profit of Believing (New Advent)