Paul Earnhart on Matthew 5:17-20 (emphasis added):
It would be exceedingly helpful if students of the Scripture could realize that God’s law or will for man inheres in creation, not in the covenants. The Creator’s expectations for His creature, man, have been in place since Adam. The two greatest commandments (Deuteronomy 6:5; Leviticus 19:18; Mark 12:28-31) did not first have application when they were included in the covenant made with Israel at Sinai, but were clearly applied to man’s behavior toward God and others from his inception (Genesis 4:1-12; 6:5,11-13; 18:20; Jude 7). A man is not under law to God because he is under covenant (new or old). He is under law to God because he is a man. One can only escape God’s law by resigning from the human race. Resignations have frequently been tendered, but there is no evidence that any have been accepted. Man, under covenant, makes a commitment to be faithful to God and His commands and receives in turn the promises and blessings of the Lord—but whatever he does there is no escape from divine law.
But why, it is asked, does Jesus, in a sermon on the “gospel of the kingdom,” urge upon His hearers the careful keeping of the least commandment of the law of Moses? The answer is: because His audience was Jewish and were, even as Jesus spoke, under that covenant. Whatever attitude they had toward God’s law as expressed in the Jewish covenant they were bound to bring to the kingdom. the covenant is not so important as the principle of absolute trust and obedience toward God in everything. Anyone disposed to play fast and loose with the smallest command of God, whatever the covenant, is unfit for the kingdom of heaven. A new covenant would come but the principle would remain the same.
Some ordinances of God are manifestly greater than others because they sit closer to the heart of divine righteousness (Mark 12:28-33; Matthew 23:23), but no command of God is without immense significance since the breath of the Almighty is in it (2 Timothy 3:16). The one who rebuked the Pharisees for swallowing camels did not encourage them to eat gnats with relish (Matthew 23:23). James has sought to make us understand that the commands of God are indivisible since He is behind them all (James 2:10-11). It is not just a matter of breaking a command, little or great. It is a matter of defying God and breaking faith with Him.
Obedience is not limited as a principle to systems of justification by law (Galatians 3:10). It is also an expression of faith (James 2:14-26) and love (John 14:15, 23-24; 1 John 5:3) in the gospel system of grace and justification by faith (Matthew 7:21). As such it has application to salvation in every dispensation (Hebrews 11). The kingdom citizen, like the faithful of all ages, is not seeking to justify himself by his earnest obedience to all God’s commands, but to return the love which has been poured out upon him so undeservedly. The law of God is a dagger to the heart of the arrogant and self-righteous, but to the Christian it is the standard of righteous conduct to which, under the grace of God, he aspires (Romans 12:1-2). God intends not only to redeem His people but to transform them as well (Romans 8:29; 2 Corinthians 3:18).
Invitation to a Spiritual Revolution: Studies in the Sermon on the Mount, (pg. 33-35)