“Legalism, theologically understood, can manifest itself in a variety of ways. Whether or not the Pharisees explicitly taught a merit system [….], we must recognize that Jesus is never represented in the Gospels as criticizing them for believing that they could atone for their own sins. He does indeed condemn them for their legalism—
but a legalism that finds expression in a somewhat different form, namely, through the relaxation of God’s standards
“This point can be illustrated most clearly by referring to a well-known legal ruling, the prozbul, attributed to Hillel the Elder, who apparently lived during the reign of Herod the Great. This ruling in effect did away with the command that debts were to be cancelled every seven years (Dt 15:1-3). That command was accompanied by a solem warning: ‘Be careful not to harbor this wicked thought: “The seventh year, the year for cancelling debts, is near,” so that you do not show ill will toward your needy brother and give him nothing. He may then appeal to the Lord against you, and you will be found guilty of sin” (v 9). During Hillel’s time, however, the wealthy were in fact refusing to lend money, fearing they would lose it in the sabbatical year. Since the poor were the ones suffering, Hillel (if we may trust the rabbinic attribution) used the legal fiction that debts cease to be private when transferred to a court, and he ordained that in such cases the debts may be collected. For humanitarian reasons, therefore, Hillel devised a way of ‘breaking’ the Torah; the explanation, of course, would have been that such ‘innovations and amendments . . . fulfilled the basic reason of the commandment, whereas its literal observance nullified its original intent.’ [E. E. Urbach, The Sages, 1:373. -ed.]
“This enactment—and other examples could be used—show[s] that we miss the point when we view the Pharisees as being concerned with the letter rather than the spirit of the Law. While that may well have been the case in some instances, it does not address the basic motivation for the rabbinic interpretation of the Torah. [….] In a very important sense, the Pharisees made the Torah easier to obey. As a result of the prozbul, wealthy Jews no longer needed to be concerned about the solemn warning of Deuteronomy 15:9. The divine standard had been relaxed. The Torah had been accomodated to meet the weaknesses of the people. [….]
“It turns out, then, that Jesus, who like the Old Testament prophets demanded perfection (Mt 5:48), would have been critical of the Pharisees, not because they obeyed the Torah too strictly, but because they interpreted it too loosely. This is clearly and precisely the point of Mark 7:1-13, generally recognized as a key passage for understanding the conflict between Jesus and the Pharisees. The controversy described in this passage centers on the law that ceremonial washing was required before eating. In fact, this is not an Old Testament law; it is not part of the Written Torah. But it was part of the Oral Torah, that is, the traditions of the elders. Scholars are generally agreed that the concept of the Twofold Law was the most distinctive feature of Pharisaic and later Judaism. The Oral Law was viewed as on a par with the Written Law—indeed, in some respects, as more important, for a ruling that is part of the Oral Law may in effect set aside the Written Law, as in the case of the prozbul.
“Jesus’ response to the Pharisees in Mark 7 is that they ‘have let go of the commands of God and are holding on to the tradition of men’ (v 8). And, after describing a particularly insidious example, He concludes: ‘Thus you nullify the word of God by your tradition that you have handed down’ (v 13). This undermining of God’s Word, moreover, resulted in a muted consciousness of sin, for normally there were ways of interpreting the divine commands that mitigated their force. This frame of mind is almost surely the background for Matthew 5, whre Jesus is said to demand of His disciples a righteousness greater than that of the Pharisees (v 20). Then, to preclude any interpretive moves that might render the law innocuous, He goes on to intensify specific Scriptural commands. Just in case anyone might have missed the point, He concludes: ‘Be perfect, therefore, as your heavenly Father is perfect’ (v 48), the equivalent of Leviticus 11:45, ‘ . . . therefore be holy, because I am holy.’
“The Pharisees were often in danger of thinking that they had adequately fulfilled their duty before God (cf. Lk 18:9-12:21), and therefore no great sense of dependence on God’s grace was likely to arise. In contrast, Jesus emphasized that the truse servants of God are those who are ever conscious of their unworthiness (Lk 17:7-10) and who have learned to pray, ‘God, have mercy on me, a sinner’ (Lk 18:13).”
(Moisés Silva, “The Place of Historical Reconstruction in New Testament Criticism,” in D. A. Carson and John D. Woobridge [eds.], Hermeneutics, Authority, and Canon [1986; reprint, Grand Rapids: Baker, 1995], pages 119-121.)