Objection 1. It would seem that the New Law is more burdensome than the Old. For Chrysostom (Opus Imp. in Matth., Hom. x [The work of an unknown author]) say: "The commandments given to Moses are easy to obey: Thou shalt not kill; Thou shalt not commit adultery: but the commandments of Christ are difficult to accomplish, for instance: Thou shalt not give way to anger, or to lust." Therefore the New Law is more burdensome than the Old.
Objection 2. Further, it is easier to make use of earthly prosperity than to suffer tribulations. But in the Old Testament observance of the Law was followed by temporal prosperity, as may be gathered from Deuteronomy 28:1-14; whereas many kinds of trouble ensue to those who observe the New Law, as stated in 2 Corinthians 6:4-10: "Let us exhibit ourselves as the ministers of God, in much patience, in tribulation, in necessities, in distresses," etc. Therefore the New Law is more burdensome than the Old.
Objection 3. The more one has to do, the more difficult it is. But the New Law is something added to the Old. For the Old Law forbade perjury, while the New Law proscribed even swearing: the Old Law forbade a man to cast off his wife without a bill of divorce, while the New Law forbade divorce altogether; as is clearly stated in Matthew 5:31, seqq., according to Augustine's expounding. Therefore the New Law is more burdensome than the Old.
On the contrary, It is written (Matthew 11:28): "Come to Me, all you that labor and are burdened": which words are expounded by Hilary thus: "He calls to Himself all those that labor under the difficulty of observing the Law, and are burdened with the sins of this world." And further on He says of the yoke of the Gospel: "For My yoke is sweet and My burden light." Therefore the New Law is a lighter burden than the Old.
I answer that, A twofold difficult may attach to works of virtue with which the precepts of the Law are concerned. One is on the part of the outward works, which of themselves are, in a way, difficult and burdensome. And in this respect the Old Law is a much heavier burden than the New: since the Old Law by its numerous ceremonies prescribed many more outward acts than the New Law, which, in the teaching of Christ and the apostles, added very few precepts to those of the natural law; although afterwards some were added, through being instituted by the holy Fathers. Even in these Augustine says that moderation should be observed, lest good conduct should become a burden to the faithful. For he says in reply to the queries of Januarius (Ep. lv) that, "whereas God in His mercy wished religion to be a free service rendered by the public solemnization of a small number of most manifest sacraments, certain persons make it a slave's burden; so much so that the state of the Jews who were subject to the sacraments of the Law, and not to the presumptuous devices of man, was more tolerable."
The other difficulty attaches to works of virtue as to interior acts: for instance, that a virtuous deed be done with promptitude and pleasure. It is this difficulty that virtue solves: because to act thus is difficult for a man without virtue: but through virtue it becomes easy for him. In this respect the precepts of the New Law are more burdensome than those of the Old; because the New Law prohibits certain interior movements of the soul, which were not expressly forbidden in the Old Law in all cases, although they were forbidden in some, without, however, any punishment being attached to the prohibition. Now this is very difficult to a man without virtue: thus even the Philosopher states (Ethic. v, 9) that it is easy to do what a righteous man does; but that to do it in the same way, viz. with pleasure and promptitude, is difficult to a man who is not righteous. Accordingly we read also (1 John 5:3) that "His commandments are not heavy": which words Augustine expounds by saying that "they are not heavy to the man that loveth; whereas they are a burden to him that loveth not."
Reply to Objection 1. The passage quoted speaks expressly of the difficulty of the New Law as to the deliberate curbing of interior movements.
Reply to Objection 2. The tribulations suffered by those who observe the New Law are not imposed by the Law itself. Moreover they are easily borne, on account of the love in which the same Law consists: since, as Augustine says (De Verb. Dom., Serm. lxx), "love makes light and nothing of things that seem arduous and beyond our power."
Reply to Objection 3. The object of these additions to the precepts of the Old Law was to render it easier to do what it prescribed, as Augustine states [De Serm. Dom. in Monte i, 17,21; xix, 23,26. Accordingly this does not prove that the New Law is more burdensome, but rather that it is a lighter burden.
Source: Summa Theologica (New Advent)