Matthew uses Jesus' words in 5:17-20 as a thesis statement for the whole of 5:21-48 which follows. Jesus essentially says, "Look, if you thought the law was tough, wait till you see this. If you really want to be my disciples, give me your hearts without reservation" (see 5:17).
This passage seems to suggest that an uncommitted Christian is not a Christian at all (see 5:20). Like other Jewish teachers, Jesus demanded whole obedience to the Scriptures (5:18-19); unlike most of his contemporaries, however, he was not satisfied with the performance of scribes and Pharisees, observing that this law observance fell short even of the demands of salvation (5:20). After grabbing his hearers' attention with such a statement, Jesus goes on to define God's law not simply in terms of how people behave but in terms of who they really are (5:21-48).
Jesus' High View of Scripture (5:17-18)
Jesus' view of Scripture did not simply accommodate his culture, a fact that has implications for the view of Scripture Jesus' followers should hold (J. Wenham 1977:21; D. Wenham 1979). Here Jesus responds to false charges that he and his followers undermine the law. First, when Jesus says that he came not to abolish the Law or the Prophets but to fulfill them, he uses terms that in his culture would have conveyed his faithfulness to the Scriptures (v. 17).
Second, Jesus illustrates the eternality of God's law with a popular story line from contemporary Jewish teachers (5:18). Jesus' smallest letter (NIV), or "jot" (KJV), undoubtedly refers to the Hebrew letter yod, which Jewish teachers said would not pass from the law. They said that when Sarai's name was changed to Sarah, the yod removed from her name cried out from one generation to another, protesting its removal from Scripture, until finally, when Moses changed Oshea's name to Joshua, the yod was returned to Scripture. "So you see," the teachers would say, "not even this smallest letter can pass from the Bible." Jesus makes the same point from this tradition that later rabbis did: even the smallest details of God's law are essential.
We Will Be Judged by Our Response to God's Word (5:19)
Jesus here provides a graphic example of the law's authority. Jewish teachers typically depicted various persons as "greatest" before God; the emphasis was not on numerical precision but on praising worthy people (for example, m. 'Abot 2:8). When Jesus speaks of the least of these commandments, he also reflects Jewish legal language. Jewish teachers regularly distinguished "light" and "heavy" commandments (as in Sipra VDDeho. parasha 220.127.116.11; compare Mt 23:23) and in fact determined which commandments were the "least" and "greatest." Noting that both the "greatest" commandment about honoring parents (Ex 20:12; Deut 5:16) and the "least" commandment about the bird's nest (Deut 22:6-7) included the same promise, "Do this and you will live," later rabbis decided that "live" meant "in the world to come" and concluded that God would reward equally for obedience of any commandment. One who kept the law regulating the bird's nest merited eternal life, whereas one who broke it merited damnation (see, for example, Urbach 1979:1:350; Keener 1991a:116). In the same way, those who merely honored the highest standards of their religion would fall short of entering the kingdom at all (Mt 5:20).
Other sages used such language to grab attention and emphasize the importance of the law. But like Jesus, they did not want anyone to miss the point: God has not given us the right to pick and choose among his commandments. As some teachers put it, one should be as "careful with regard to a light commandment as you would be with a heavy one, since you do not know the allotment of the reward" (m. 'Abot 2:1). The sages were not suggesting that they never broke commandments (see Moore 1971:1:467-68), but rather believed that one who cast off any commandment or principle of the law was discarding the authority of the law as a whole (m. Horayot 1:3; Keener 1991a:115-17).
Jesus concurs: God does not allow us the right to say, "I will obey his teaching about murder but not his teaching about adultery or fornication"; or, "I will obey his teaching about theft but not about divorce." To refuse his right to rule any of our ethics or behavior is to deny his lordship.
In this passage Jesus also warns that teachers who undermine students' faith in any portion of the Bible are in trouble with God. This text addresses not only obedience to the commandments but also how one teaches others (and teaches others to do the same; compare Jas 3:1). I have occasionally taught alongside colleagues who actively sought to undermine students' faith in the name of "critical thinking"; sometimes they succeeded. Critical thinking is important, but it functions best with the firm foundation of the fear of God (Prov 1:7).
Bible-Believing People Without Transformed Hearts Are Lost (5:20)
Like John the Baptist in 3:7-12, Jesus savages the false security of the religious establishment. To grasp the full impact in today's language we might compare the scribes with ministers or religious educators and the Pharisees with the most pious, Bible-believing laypeople (although there was some overlap between the two groups). Pharisaic ethics emphasized "inwardness" as much as Jesus did, but Jesus challenges not their traditional ethics but the actual condition of their hearts (Odeberg 1964).
It is possible to agree with everything Jesus taught in this sermon yet fail to live accordingly (23:3). That is why Jesus indicates that the best of human piety is inadequate for salvation-whether it be Pharisaic or Christian. Nothing short of a radical transformation, what other early Christian writers called a new birth (Jn 3:3-6; 1 Pet 1:23), can enable one to live as a disciple (compare Mt 18:3).