You are clearly misinterpreting (at best) or not understanding (at worst).
To whom was Matthew's gospel written? Clearly, it was written to the Jewish people to give them understanding of their Messiah. It was not written to Christians! When He warned against relaxing the least part of it or teaching others to relax the least part of it, that was said before the new covenant was instituted.
A Christian is by definition a follower of Christ. So is it your position that the way to follow Christ is by refusing to follow what he taught in Matthew? In Matthew 28:16-20, Jesus commissioned his disciples to make disciples of all nations, teaching everything that he taught them, so that doesn't leave room for followers of Christ to refuse to follow what he taught in Matthew. Jesus didn't establish the New Covenant for the purpose of undermining anything that he spent his ministry teaching, but rather the New Covenant still involves following the Mosaic Law (Jeremiah 31:33).
The Mosaic Law is holy, righteous, and good (Romans 7:12), so it is the law where holiness, righteousness, and goodness have dominion over us, however, the law of sin stirs up sinful passions in order to bear fruit unto death, so it is the law where sin had dominion over us, and Romans 6:14 describes the law that we are not under as being the law where sin had dominion over us, so he was referring to the law of sin, not to the Law of Moses. In Romans 6:15, being under grace does not mean that we are permitted to sin, and in Romans 3:20, it is by the Mosaic Law that we have knowledge of what sin is, so we are still under the Mosaic Law, but are not under the law of sin. In Psalms 119:29, he wanted God to be gracious to him by teaching him to obey the Mosaic Law, so that is what it means to be under grace.
Moreover, everything else in Romans 6 speaks in favor of obedience to God and against sin. For example in Romans 6:16-23, we are slaves to the one whose we obey either of sin which leads to death or to obedience, which leads to righteousness, we have been set free from sin in order to become slaves of righteousness, we should no longer present ourselves as slave to impurity, lawlessness, and sin, but should now present ourselves as slaves to God and to righteousness leading to sanctification, and the goal of sanctification is eternal life in Christ, which is the gift of God. So to reject being under God's law is to reject the content of God's gift of eternal life in Christ.
Again, Paul said that he wanted to do good, but spoke against the law of sin that was controlling him and causing him not to do the good that he wanted to do. In Romans 7:22-23, Paul delighted in obeying the Law of God, but contrasted that with the law of sin that held him captive, and Romans 7:6 says that we have been released from a law that held us captive, so he was referring to being released from the law of sin. We need to die to the law of sin in order to be free to obey the Law of God, not the other way around.
The Mosaic Law is God's word and Jesus is God's word made flesh, so why does it make sense to you to think that we are denying Christ by obeying God's word? Would it make more sense. to think that the way to deny Christ is by refusing to obey God's word? Christ set a sinless example of how to walk in obedience to the Mosaic Law, so why does it make sense to you to think that the way to deny Christ is by following his example instead of by refusing to follow him?[/QUOTE]
Clearly,
you do not understand the New Covenant. If people, including Christians, are still under the OT law, then Jesus Christ died for nothing!