1. The affirmation Christ made during the Sermon on the Mount regarding adultery and desire, which he called "adultery of the heart," must be analyzed from the very beginning. Christ said: "You have understood that it was said: 'You shall not commit adultery'" (Mt 5:27). He had in mind God's commandment, the sixth in the Decalogue, included in the so-called second Table of the Law which Moses received from God-Yahweh.
First of all, let's place ourselves in the situation of the audience present during the Sermon on the Mount, those who actually heard the words of Christ. They are sons and daughters of the chosen people—people who had received the law from God—Yahweh himself. These people had also received the prophets. Time and time again throughout the centuries, the prophets had reproved the people's behavior regarding this commandment, and the way in which it was continually broken. Christ also speaks of similar transgressions. But he speaks more precisely about a certain human interpretation of the law, which negates and does away with the correct meaning of right and wrong as specified by the will of the divine legislator. Above all, the law is a means—an indispensable means if "justice is to abound" (Mt 5:20). Christ desires such justice to be "superior to that of the scribes and Pharisees." He does not accept the interpretation they gave to the authentic content of the law through the centuries. In a certain way, this interpretation subjected this content, or rather the purpose and will of the legislator, to the varied weaknesses and limits of human willpower deriving precisely from the threefold concupiscence. This was a casuistic interpretation which was superimposed on the original version of right and wrong connected with the law of the Decalogue. If Christ tends to transform the ethos, he does so mainly to recover the fundamental clarity of the interpretation: "Do not think that I have come to abolish the law or the prophets; I have not come to abolish but to fulfill" (Mt 5:17). Fulfillment is conditioned by a correct understanding, and this is applied, among others, also to the commandment: "You shall not commit adultery."
2. Those who follow the history of the chosen people in the Old Testament from the time of Abraham will find many facts which witness to how this commandment was put into practice. As a result of such practice, the casuistic interpretation of the law developed. First, it is well known that the history of the Old Testament is the scene for the systematic defection from monogamy. This fact must have a fundamental significance in our understanding of the prohibition: "You shall not commit adultery." Especially at the time of the patriarchs, the abandonment of monogamy was dictated by the desire for offspring, a very numerous offspring. This desire was very profound, and procreation as the essential end of marriage was very evident. This was so much so that wives who loved their husbands but were not able to give them children, on their own initiative asked their husbands who loved them, if they could carry "on their own knees," or welcome, his children born of another woman, for example, those of the serving woman, the slave. Such was the case of Sarah regarding Abraham (cf. Gn 16:2) or the case of Rachel and Jacob (cf. Gn 30:3). These two narratives reflect the moral atmosphere in which the Decalogue was practiced. They illustrate the way in which the Israelite ethos was prepared to receive the commandment, "You shall not commit adultery," and how such a commandment was applied in the most ancient tradition of this people. The authority of the patriarchs was the highest in Israel and had a religious character. It was strictly bound to the covenant and to the promise.
The commandment, "You shall not commit adultery," did not change this tradition. Everything points to the fact that its further development was not limited by the motives (however exceptional) which had guided the behavior of Abraham and Sarah, or of Jacob and Rachel. For example, the lives of the most renowned Israelites after Moses, the kings of Israel, David and Solomon, show the establishing of real polygamy, which was undoubtedly for reasons of concupiscence.
3. In the history of David, who also had other wives, we are struck not only by the fact that he had taken the wife of one of his subjects, but also by the fact that he was clearly aware of having committed adultery. This fact, as well as the king's repentance, is described in a detailed and evocative way (cf. 2 Sm 11:2-27). Adultery is understood to mean only the possession of another man's wife, but it is not considered to be the possession of other women as wives together with the first one. All Old Testament tradition indicates that the real need for monogamy as an essential and indispensable implication of the commandment, "You shall not commit adultery," never reached the conscience and the ethos of the following generations of the chosen people.
Against this background one must also understand all the efforts which aim at putting the specific content of the commandment, "You shall not commit adultery," within the framework of the promulgated laws. It is confirmed by the books of the Bible in which we find the Old Testament legislation fully recorded as a whole. If we consider the letter of such legislation, we find that it takes a determined and open stand against adultery, using radical means, including the death penalty (cf. Lv 20:10; Dt 22:22). It does so, however, by effectively supporting polygamy, even fully legalizing it, at least indirectly. Therefore, adultery was opposed only within special limits and within the sphere of definitive premises which make up the essential form of the Old Testament ethos. Adultery is understood above all (and perhaps exclusively) as the violation of man's right of possession regarding each woman who may be his own legal wife (usually, one among many). On the contrary, adultery is not understood as it appears from the point of view of monogamy as established by the Creator. We know now that Christ referred to the "beginning" precisely in regard to this argument (Mt 19:8).
4. Furthermore, the occasion in which Christ took the side of the woman caught in adultery and defended her from being stoned to death is most significant. He said to the accusers: "Whoever of you is without sin, let him throw the first stone" (Jn 3:7). When they put down the stones and went away, he said to the woman: "Go, and from now on, sin no more" (Jn 8:11). Therefore, Christ clearly identified adultery with sin. On the other hand, when he turned to those who wanted to stone the adulteress, he did not refer to the precepts of Israel's law but exclusively to conscience. The discernment between right and wrong engraved on the human conscience can show itself to be deeper and more correct than the content of a norm.
As we have seen, the history of God's people in the Old Testament (which we have tried to illustrate through only a few examples) took place mainly outside the normative content contained in God's commandment, "You shall not commit adultery." It went along, so to speak, side by side with it. Christ wanted to straighten out these errors, and thus we have his words spoken during the Sermon on the Mount.
Source: L'Osservatore Romano