Just read the following on the Sermon on the Mount, regarding whether the fact that Christians fall short of its standards destroys the point of the Sermon:
A father with carpentry skills decides to build a shed. The task presents an opportunity to spend some “quality time” with his eight-year-old twin boys and perhaps to teach them a thing or two about carpentry. He invites their participation. Both are excited, but, though Johnny agrees to help, Jimmy decides he would rather build a shed on his own. They set to work. Tommy, the boys’ friend, drops by and is immediately impressed by Jimmy’s activities, hammering and sawing all on his own, with what appears to Tommy to be considerable skill. Johnny, by comparison, appears positively awkward and quite unproductive in all he does—bringing a hammer to his dad; driving in nails with his dad’s hand also on the hammer, occasionally attempting a few strokes on his own, but as often as not having his father pull out and straighten the nails he has hit. Tommy can only conclude that Jimmy is much the better carpenter.
But Johnny and his dad produce a shed, and a fine one at that (his father is a good carpenter as well as a devoted dad). Jimmy produces a mess. The fact is, Jimmy and Johnny are both a decade or more away from being able to build a shed. Still, Johnny has now had a “part” in the making of one and, for all his awkwardness and misguided strokes, learned something about carpentry in the process. Jimmy got Nowhere and learned nothing (beyond, one would hope, his own limitations).
Doubtless Matthew and Paul saw God’s righteousness and goodness as lying as far beyond human capacities as the building of a shed is beyond the skills of an eight-year-old. External observers may be impressed by any number of virtuous deeds on the part of “Jimmy’s” kin; but, from this perspective, they amount to little. Human virtue unaided will never take on the character of divine goodness. The latter can only be produced by “cooperation” with God. Matthew and Paul saw followers of Jesus as Johnny’s kin, and their assurance that God’s righteousness would result from such “Johnnies'” endeavors had nothing to do with virtues they perceived in God’s little “helpers.” Where a child is eager and willing to help, a competent dad will see to it that the job gets done.
From Stephen Westerholm: “The Law in the Sermon on the Mount: Matt 5:17-20, Criswell Theological Review, 1992, p. 51-52.