Within the nations of Israel, rules had to be enacted to place some restraint on less than perfect situations. Divorce was inevitable, so the woman who became its victim might at least receive a certificate indicating her unmarried status so that she could marry again. Violance was unavoidable, so retribution for the injury one had incurred might at least be limited to nothing more harmful than what one had experienced – an eye (and no more) for an eye; a tooth (and no more) for a tooth. The “law of the LORD” was indeed “perfect” for these less than perfect situations in a theocracy that included both the godly and those whose hearts were corrupt.
In contrast, Matthew believed that Jesus was assembling a new people who were “pure in heart” (5:8). For such a people the humane foundation that lay beneath the Mosaic law could be brought to the surface and the Mosaic law brought to its fulfillment. In the situation Jesus envisioned, the only court would be the eschatological judgment of God, and the maximum punishment would not be physical death but hell itself (5:22, 29-30). Evidence in this court would not be outward, physical violations of normal societal statutes but the intentions of the heart (5:22, 28; cf. 6:21; 12:34; 13:15; 15:8, 18; 19:8)
Since Jesus did not define this new people as a political entity, the only standard that mattered would be God’s ultimate standard. There should not only be no murder, but none of the hate-filled anger that produces murder (5:21-22). There should not only be no adultery, but there should be no lust, which leads to adultery (5:27-30). Marriage should be the institutionalization of the permanent bonding of two people into one flesh through sexual intercourse, and one’s marriage should only be declared a failure if one’s spouse was sexually unfaithful (19:3-9; cf. 5:31-32). Disciples of Jesus must not merely limit to a reasonable level of vengeance that they take against those who harm them, but they must do their enemies no violance at all (5:38-42).
This is the sense, therefore, in which Jesus fulfilled the law and in which none of it passed away in his teaching. The Mosaic law had legislated love for God and neighbor in the less than perfect situation of a theocraacy. With the coming of Jesus, God’s law could be reduced to its fundamental principles since Jesus’ disciples were called upon to “be perfect…as your heavenly Father is perfect” (5:48). (Thielman, Frank, Theology of the New Testament, pg. 88-89)