How ought we to understand and utilize the morality found in the Old Testament? Critics of Christianity and Judaism have long pointed out certain difficult passages, but these passages can often be explained in such a way that, if not convincing the skeptic, can at least keep the parish from revolting. But then we have the Church Fathers who say that the New Testament morality is superior. Not that the Mosaic law was said to be defective, but rather that in the new covenant we have loftier goals. For example, St. Augustine said:
By the law of works, then, the Lord says, "Thou shalt not covet:" (Ex. 20:17; Deut. 5:21) but by the law of faith He says, "Without me ye can do nothing;" (Jn. 15:5) for He was treating of good works, even the fruit of the vine-branches. It is therefore apparent what difference there is between the old covenant and the new,--that in the former the law is written on tables, while in the latter on hearts; so that what in the one alarms from without, in the other delights from within; and in the former man becomes a transgressor through the letter that kills, in the other a lover through the life-giving spirit.
-- St. Augustine, A Treatise on the Spirit and the Letter, 42
Again, it is asked whether, if, with a wife's permission, either a barren one, or one who does not wish to submit to intercourse, a man shall take to himself another woman, not another man's wife, nor one separated from her husband, he can do so without being chargeable with fornication? And an example is found in the Old Testament history; but now there are greater precepts which the human race has reached after having passed that stage; and those matters are to be investigated for the purpose of distinguishing the ages of the dispensation of that divine providence which assists the human race in the most orderly way…
-- St. Augustine, On the Sermon on the Mount, 1, 16
And we find similar passages in St. John Chrysostom, such as when he speaks of the moral teachings of Jesus, saying of them: "the injunctions [are] high, and far surpassing those in the Old Testament" (Homily 15 on Matthew). And St. John, in explaining the meaning of the verse: "Think not that I have come to destroy the Law or the Prophets" (Matt. 5:17), explains that the "things which He was about to introduce were a sort of addition, not however lessening, but enhancing virtue" (Homily 16 on Matthew). He continues later in the Homily:
And how, one may ask, did He not destroy it? In what way did He rather fulfill either the law or the prophets?
…But if any one will inquire accurately, he will find also another, a third sense, in which this has been done. Of what sort is it then? In the sense of that future code of laws, which He was about to deliver to them.
For His sayings were no repeal of the former, but a drawing out, and filling up of them. Thus, “not to kill,” is not annulled by the saying, Be not angry, but rather is filled up and put in greater security: and so of all the others.
Wherefore, you see, as He had before unsuspectedly cast the seeds of this teaching; so at the time when from His comparison of the old and new commandments, He would be more distinctly suspected of placing them in opposition, He used His corrective beforehand. For in a covert way He had indeed already scattered those seeds, by what He had said. Thus, “Blessed are the poor,” is the same as that we are not to be angry; and, “Blessed are the pure in heart,” as not to “look upon a woman for lust;” and the “not laying up treasures on earth,” harmonizes with, “Blessed are the merciful;” and “to mourn” also, “to be persecuted” and “reviled,” coincide with “entering in at the strait gate;” and, “to hunger and thirst after righteousness,” is nothing else than that which He says afterwards, “Whatsoever ye would that men should do to you, do ye also to them.” And having declared “the peace-maker blessed,” He again almost said the same, when He gave command “to leave the gift,” and hasten to reconciliation with him that was grieved, and about “agreeing with our adversary.”
But there He set down the rewards of them that do right, here rather the punishments of them who neglect practice. Wherefore as in that place He said, “The meek shall inherit earth;” so here, “He who calls his brother fool, shall be in danger of hell-fire;” and there, “The pure in heart shall see God;” here, he is a complete adulterer who looks unchastely. And having there called “the peace-makers, sons of God;” here He alarms us from another quarter, saying, “Lest at any time the adversary deliver you to the judge.” Thus also, whereas in the former part He blesses them that mourn, and them that are persecuted; in the following, establishing the very same point, He threatens destruction to them that go not that way; for, “They that walk 'in the broad way,' says He, 'make their end there.'” And, “You cannot serve God and mammon,” seems to me the same with, “Blessed are the merciful,” and, “those that hunger after righteousness.”
But as I said, since He is going to say these things more clearly, and not only more clearly, but also to add again more than had been already said (for He no longer merely seeks a merciful man, but bids us give up even our coat; not simply a meek person, but to turn also the other cheek to him that would smite us): therefore He first takes away the apparent contradiction.
On this account, then, as I have already stated, He said this not once only, but once and again; in that to the words, “Think not that I have come to destroy,” He added, “I am not come to destroy, but to fulfill.”
“For verily I say unto you, Till Heaven and earth pass, one jot or one tittle shall in no wise pass from the law, till all come to pass.”
Now what He says is like this: it cannot be that it should remain unaccomplished, but the very least thing therein must needs be fulfilled. Which thing He Himself performed, in that He completed it with all exactness.
And here He signifies to us obscurely that the fashion of the whole world is also being changed. Nor did He set it down without purpose, but in order to arouse the hearer, and indicate, that He was with just cause introducing another discipline; if at least the very works of the creation are all to be transformed, and mankind is to be called to another country, and to a higher way of practising how to live.
-- St. John Chrysostom, Homily 16 on Matthew
Where does this leave us? Has all Old Testament morality been superceded? Many Christians make a fuss about the Ten Commandments, and one would assume that it can be considered part of “a higher way of practicing how to live”… but where do we draw the line? How do we know when we are ignoring an Old Testament passage because it doesn’t sit well with our modern sensibilities, as opposed to it actually be superceded by a loftier goal? The simple answers are "Trust the Church" and "Look to the Fathers" or "Consult the New Testament", and so forth, but there are sometimes disagreements about what exactly the Church or the Fathers or even the black-and-white New Testament says.