I probably cannot change Soyeong's concepts, but I can address the problems with those.
In Matthew 4:15-23, it describes the Gospel that Jesus taught as the message to repent the Kingdom of God is at hand, which was a light to the Gentiles, and the Mosaic Law was how his audience knew what sin is, so I don't see how you can claim that it was a message given to these guys to be Christ's disciples or diminish the relevance of this passage to the Gospel of the Kingdom.
About Mat 4:15-23
That passage is very narrow. Only Matt 4:15 could possibly extend to gentiles, but that does not put them under the Mosaic law. Also, the kingdom of God was of interest to the Jews. If this had been preached to the gentiles, they would simply have been confused by the idea.
Indeed, Jesus did not establish the New Covenant until the end of his ministry, which means that everything that he taught prior to that point was in regard to how to live under the Mosaic Covenant and is cumulatively relevant to how we should live under the New Covenant. While Jesus focused his ministry on Jews, he commissioned his disciples to bring the Gospel of the Kingdom to the Gentiles and to teach the Gentiles everything that he taught them.
About the Great Commission...
False. He commissioned the 12 to go to the Jewish nations. They did not realize that gentiles were included until Acts 10. I know people misuse Matt 28:19-20 because they do not read it carefully, nor in the Jewish context of Matthew.
The whole can't be held accountable to God by His law if the whole world is not under it. If Gentiles were not obligated God's law, then they would have no need to believe the Gospel of the Kingdom that calls for our repentance from our disobedience to it, they would have no need of Jesus to have given himself to redeem us from all lawlessness, and they would have no need for salvation from transgressing it.
I understand your confusion of Rom 3:19. However, the baseline to read Romans 3 is that gentiles knew they were considered sinners. So, Rom 3:9-20 was to show that Jews were no closer to righteousness through the law than gentiles were without being under the law. You have learned a common error of reading Romans, so you just need to skip using this passage.
The Kingdom of God is composed of citizens who are followers of the Law of God in accordance with the Gospel message to repent for the Kingdom of God is at hand. Christ spent his ministry teaching his followers to obey the Law of God by word and by example and being a Christian is about being a follower of what he taught. While Gentiles don't need to become Jews in order to become followers of Christ, Gentiles can't follow Christ by refusing to follow what he taught. If you're taking the position that Gentiles coming to Christ aren't turning away from the Law of God, then I'm not sure why you are also taking the position that Gentile Christians are not subject to it.
Regarding Kingdom of God ...
I have no idea where you get that summation of the kingdom of God. oh well.
Jesus spoke to Jews who were under the Mosaic law and even over-focused on it, as if they could get righteous by it. I try to imagine which verses you use for your view on this. I repeat Rom 3:19, which was to tell the gentiles the Mosaic law was not for them.
The law just got distorted and made the focus of their practices. In reality, it was code covering criminal law, civil law, priestly law, health practices, and festival (or cultural) elements. It would not have relevance to gentiles. Jesus did not teach to do the law but he shared problems caused by the Jewish creation of new laws and of judging each other.
The God of Israel has given instructions through His law for how to know, love, worship, glorify, believe in, and testify about Him by embodying His likeness through being a doer of His character traits, so if your goal is to do those things, then you will voluntarily follow God's instructions for how to do them rather than try to say that you are not subject to them.
About God's supposed instruction to "love, worship" ...
Paul shares that laws were not given on good things to do. (Sure, there are some exceptions -- like returning lost items and gleaning laws.) There are no instructions to give a shirt to someone who has none or to be hospitable, etc...
If there are good things to do and are volunteered action, that is fine to do. If someone is unrighteous (i.e. lacking Christ), they will have to have a law over them.
So then are you taking the poison that new covenants do nullify the promises of covenants that have already been ratified so as to making the promises of God void?
Regarding covenants, Isarel, and gentiles ....
I have explained Gal 3:16-20. I cannot keep explaining it and expecting you to correct your wrong belief. Okay here is something...
The Mosaic covenant was not broken or nullified. It was completed. It was a covenant to the Israel people. If a Jew had decided to stay under that instead of the better covenant through Christ, they could have done that -- and probably suffer the consequences.
Gentiles never were under the Mosaic covenant, so being in Christ only brought gentiles to the Abrahamic covenant. I hope this point helps.
While the promise is not earned as the result of our obedience to God's law, that does not mean that the content of what is promised is not in regard to God's law. The Gospel that Jesus taught in Matthew 4:15-23 that called for our repentance form our disobedience to God's law is in accordance with him being sent as the promised seed to bless us by turning us from our wickedness, which is the Gospel that was made known in advance to Abraham in accordance with the promise (Galatians 3:8), and which he spread to Gentiles in Haran in accordance with the promise (Genesis 12:1-5).
Again about Matt 4:15-23...
You have sort of gone around in a circle here. If you think your sins are still accumulating, you need to reevaluate the promises in Christ. If you are thinking you are a Christian while murdering people -- David had done that and still was with God, but that is not permission to do the same.
I have no idea what you mean about Abraham sharing any message of God with people in Haran. Is that some sort of Christian mythology?
In Genesis 18:19, God knew Abraham that he would teach his children and those of his household to walk in God's way by being doers of righteousness and justice that the Lord might bring to him all that He has promised. In Genesis 26:4-5, God will multiple Abraham's children as the stars in the heavens, to his children He will give all of these lands, and through his children all of the nations of the earth shall be blessed because he heard God's voice and guarded His charge, commandments, statutes, and laws. In Deuteronomy 30:16, if the children of Abraham will love God with all of their heart by walking in His way in obedience to His commandments, statutes, and laws, then they will live and multiply and God will bless them in the land that they go to possess. So the promises was made to Abraham and brought about because he walked in God's way in obedience to His law, he taught his children and those of his household to do that in accordance with spreading the Gospel of the Kingdom, and because they did that.
Regarding Gen 18,26 and Deut 30 ...
The works did not involve the law. The action required was to accept the promised Messiah. The idea that Abraham followed the Mosaic law was Jewish exaltation of the criminal code shared to them by Moses.
Abraham had no law of God except to leave his home for the promised land. The blessings promise was not fulfilled until Christ, so I'm not sure what you think Abraham taught Isaac.
In John 8:39, Jesus said that if they were children of Abraham, then they would be doing the same works as Abraham, so the way that the children of Abraham and multiplied and are a blessing to the nations in accordance with inheriting the promise through faith is by turning the nations from their wickedness and by teaching them to do the same works as Abraham by walking in God's way in obedience to His law, and the position that God's law is just for Jews is missing the whole point of the promise and of the Gospel.
Regarding John 8:39 incident ...
I do not think Jesus was telling them they failed to be a blessing to the nations. His issue was their rejection of himself -- to whatever they thought he was. I think many recognized him as a possible prophet though.
You really do exalt the law mentality.
Again, being a Christian is about being a follower of what Christ taught. The laws against theft, adultery, and murder are part of the Mosaic Law. If someone does not have the freedom to do that the Mosaic Law reveals to be wickedness, then there would be no point in them arguing that they aren't under it. If someone holds the position that we are no longer under the Mosaic Law now that Christ has come, then there is no point them also holding the position that someone becoming a Christians doesn't mean that they will suddenly cease to obey it.
About Mosaic law against theft, adultery ...
Since gentiles are not under the Mosaic criminal code, they are not obligated to it. They likely will not violate the criminal code of the countries they live in. While it can be said that God gave much important concepts of criminal code, it does not mean that all people are under the conditions of the Mosaic law. In fact, no places or people actually even follow the processes and punishments specifically given in the Mosaic law. Also, if a Christian sins, he is forgiven through Christ.
And my point about people not suddenly becoming murderers upon following Christ, I share that against the fear of people somehow becoming wicked upon following Christ, as if not being under the Mosaic law will inspire them to be wicked.
While we do not earn eternal life as the result of our obedience to God's law, there are many verses that making abundantly clear that the way to have eternal life is obeying it. For example, in Romans 2:6-7, Paul said that those who persist in doing good will be given eternal life. In Luke 10:25-28, Jesus affirmed that the way to inherit the gift of eternal life is by obeying the greatest two commandments of the Mosaic Law.
I'm curious whether you were taught this broad view of your post or something you came up with on your own.
Rom 2:6-7 is about good works rather than works of the law. That is about showing love, not just avoiding murder of a neighbor each day.
Luke 10:25-28 is followed by the parable of the good Samaritan. It was showing that man's claim to follow the law was off track since the important point was to show mercy. Even if this man followed the law, his heart was wicked, especially as seen by his testing of Jesus.
Jesus was speaking as a teacher within Judaism of that day. Actually the part that Jesus endorsed was loving God and loving one's neighbor, not the letter of the law.
In Hebrews 5:9, Jesus has become a source of eternal salvation for those who obey Him. In Romans 6:19-23, we are no longer to present ourselves as slaves to impurity, lawlessness, and sin, but are now to present ourselves as slaves to God and to righteousness leading to sanctification and the goal of sanctification is eternal life is Christ, which is the gift of God, so being a doer of the Mosaic Law is His gift of eternal life.
Jesus gave us the law of love. Indeed obey that. I'm not sure why that is a problem.
Rom 6:19-23 is to free the gentiles who got caught up in the fleshly behavior. He did not try to tell them to obey the Mosaic law. Your over-zealous focus on the Mosaic law led you to misread this passage.
I should add that anyone who thinks he is following the Mosaic law as a Christians is totally mistaken in that view. Unless you are stoning people caught in extra-marital sex (as one example), you are not following it.
Hope this all helps.