Objection 1. It would seem that the New Law is not contained in the Old. Because the New Law consists chiefly in faith: wherefore it is called the "law of faith" (Romans 3:27). But many points of faith are set forth in the New Law, which are not contained in the Old. Therefore the New Law is not contained in the Old.
Objection 2. Further, a gloss says on Matthew 5:19, "He that shall break one of these least commandments," that the lesser commandments are those of the Law, and the greater commandments, those contained in the Gospel. Now the greater cannot be contained in the lesser. Therefore the New Law is not contained in the Old.
Objection 3. Further, who holds the container holds the contents. If, therefore, the New Law is contained in the Old, it follows that whoever had the Old Law had the New: so that it was superfluous to give men a New Law when once they had the Old. Therefore the New Law is not contained in the Old.
On the contrary, As expressed in Ezekiel 1:16, there was "a wheel in the midst of a wheel," i.e. "the New Testament within the Old," according to Gregory's exposition.
I answer that, One thing may be contained in another in two ways. First, actually; as a located thing is in a place. Secondly, virtually; as an effect in its cause, or as the complement in that which is incomplete; thus a genus contains its species, and a seed contains the whole tree, virtually. It is in this way that the New Law is contained in the Old: for it has been stated (1) that the New Law is compared to the Old as perfect to imperfect. Hence Chrysostom, expounding Mark 4:28, "The earth of itself bringeth forth fruit, first the blade, then the ear, afterwards the full corn in the ear," expresses himself as follows: "He brought forth first the blade, i.e. the Law of Nature; then the ear, i.e. the Law of Moses; lastly, the full corn, i.e. the Law of the Gospel." Hence then the New Law is in the Old as the corn in the ear.
Reply to Objection 1. Whatsoever is set down in the New Testament explicitly and openly as a point of faith, is contained in the Old Testament as a matter of belief, but implicitly, under a figure. And accordingly, even as to those things which we are bound to believe, the New Law is contained in the Old.
Reply to Objection 2. The precepts of the New Law are said to be greater than those of the Old Law, in the point of their being set forth explicitly. But as to the substance itself of the precepts of the New Testament, they are all contained in the Old. Hence Augustine says (Contra Faust. xix, 23,28) that "nearly all Our Lord's admonitions or precepts, where He expressed Himself by saying: 'But I say unto you,' are to be found also in those ancient books. Yet, since they thought that murder was only the slaying of the human body, Our Lord declared to them that every wicked impulse to hurt our brother is to be looked on as a kind of murder." And it is in the point of declarations of this kind that the precepts of the New Law are said to be greater than those of the Old. Nothing, however, prevents the greater from being contained in the lesser virtually; just as a tree is contained in the seed.
Reply to Objection 3. What is set forth implicitly needs to be declared explicitly. Hence after the publishing of the Old Law, a New Law also had to be given.
Source: Summa Theologica (New Advent)