Gentiles are under the Abrahamic covenant of promise.
The Gospel of the Kingdom that Jesus spread in Matthew 4:15-23 that called for us to turn from our disobedience to the Mosaic Law is in accordance with him being sent as the promised seed to bless us by turning us from our wickedness (Acts 3:25-26), which is the Gospel that was made known in advance to Abraham in accordance with the promise (Galatians 3:8). The promise 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 in obedience to the Mosaic Law (Genesis 18:19, Genesis 26:4-5, Deuteronomy 30:16). The Mosaic Law is how the children of Abraham know how to be blessed by walking in God's way (Psalm 119:1-3), so the way that the children of Abraham 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 in accordance with spreading the Gospel of the Kingdom.
Revealing humanity's inability to keep the Law
In Romans 10:5-8, Paul referred to Deuteronomy 30 as the word of faith that we proclaim in regard to proclaiming that the Mosaic Law is not too difficult for us to keep and that keeping it brings life and a blessing while disobedience brings death and a curse, so choose life! So it was presented as a possibility and as a choice, not as something that we have the inability to keep.
The reference there in mat 7:23 is out of context.
I is not taking it out of context to say that the way to know Jesus is by being workers of lawfulness. In Jeremiah 9:3 and 9:6, they did not know God and refused to know Him because in 9:13 they had forsaken the Torah while in 9:24, those who know God know that he delights in being a doer of steadfast love, justice, and righteousness is also of the earth, so the way to know God is by delighting in being a doer of His character traits in obedience to the Torah. Similarly, those who say that they know Jesus but don't obey his commands are liars (1 John 2:4) and those who continue to be doers of sin in transgression of the Torah have never seen nor known him (1 John 3:4-6). So the way to know God and Jesus is embodying His character traits in obedience to the Torah, which is eternal life (John 17:3).
Jesus is the gentiles (and now Jews) sabbath rest on any day or time.
Someone can't enter into God's rest while having the same disobedience to the Mosaic Law that prevented the Israelites from entering into God's rest.
Christian do not need the mosaic law they fulfill it by the greatest commandment "love god and love thy neighbor"
Everything in the Mosaic Law is either in regard to how to love God or how to love our neighbor, so the position that we should obey the greatest two commandments is also the position that we should obey the rest of the Mosaic Law, which is why love fulfills it. Someone who was correctly living in obedience to the greatest two commandments would be indistinguishable from someone who was correctly living in obedience to the rest of the Mosaic Law because they would both be following the same example that Jesus set for us to follow.
The mosaic law was never for salvation. The mosaic law does not define all sin for all peoples.
The Mosaic Law was never given as a way of earning our salvation even as the result of having perfect obedience to it, but Jesus graciously teaching us to experience being a doer of it is intrinsically the way that he is giving us his gift of saving us from not being a doer of it. Sin is what is contrary to God's character traits and it is by the Mosaic Law that we have knowledge of what sin is because it was giving in order to teach us how to know God and Jesus through embodying His character traits, so sin is specific to who God is, not to who Israel is.
Jesus nor his disciple ever taught any gentile to follow the mosaic law.
In Matthew 28:16-20, Jesus commissioned his disciples to teach to the nations everything that he taught them.
We are to do the will of his father which is believe in Jesus.
God has made His will known through what He has commanded in the Mosaic Law (Psalm 40:8), which is also His instructions for how to believe in Jesus, which is why the Bible frequently connects our belief in God with our obedience to Him, such as with Revelation 14:12 where those who kept faith in Jesus are the same as those who kept God's commandments.
The covenant was centered around the Ten Commandments
God's character traits are eternal, so there is a reality that is true about the way to know Him through embodying His character traits that was true before God made any covenants with man, which has been revealed through His covenants, and that is true regardless of which covenant someone is under, if any.
The reason why Jesus established the New Covenant was not in order to nullify anything that he spent his ministry teaching or so that we could continue to have the same lawlessness that caused it to be needed in the first place, but rather the New Covenant is made with the same God with the same which traits and therefore still involves following the Mosaic Law (Jeremiah 31:33).
You have a major misunderstanding
In Titus 2:14, Jesus gave himself to redeem us from all lawlessness and to purify for himself a people of his own possession who are zealous for doing good works, so the way to believe in what Jesus spent his ministry teaching and in what he accomplished through the cross is by repenting and becoming zealous for doing good works in obedience to the Mosaic Law (Acts 21:20). In Psalm 119:160, all of God's righteous laws are eternal.
He fulfilled the Law and with His death
If you retroactively insert the cross back into Matthew 5 in spite of Jesus making no reference to it, then you are interpreting what it means to fulfill the law with the assumption that Jesus had no intention for his audience to understand what he meant, however, if his audience did understand what he meant, then it had meaning that his audience already understood before Jesus said that he came to fulfill it that is not is not specific to him. "To fulfill the law" means "to cause God's will (as made known in the law) to be obeyed as it should be" (NAS Greek Lexicon: pleroo), so Jesus proceeded to fulfill the law throughout the rest of Matthew 5 by correcting what the people had heard being said and by teaching how to correctly obey it as it was originally intended.
According to Galatians 5:14, anyone who has ever loved their neighbor has fulfilled the entire law, so again it refers to correctly obeying it, moreover, it refers to something that countless people have done and should continue to do in perpetuity, not to something unique that only Jesus did through the cross to remove our obligation to love our neighbor. In Galatians 6:2, bearing one another's burdens fulfills the Law of Christ, yet you do not consistently interpret that as meaning that we are no longer obligated to obey it.
That is not my position.
If I have a agreement (covenant) to write a piece of software for someone, and I complete this agreement to their satisfaction perfectly.
Abolishing or relaxing the law does not refer removing any record of its existence but to causing it to no longer be obeyed. There was nothing about God giving the Mosaic Covenant that presented it as Him wanting the Israelites to do a specific task that that is no longer required once it has been accomplished, but rather the Bible repeatedly describes the Mosaic Covenant as being eternal and uses terms like that this is a statute forever throughout your generations.
The reason that God's Word leads us to God's Word made flesh is because he is the embodiment of it and because it was given in order to teach us how to know him through following his example of embodying it, but the reason that it leads us to him is not so that we can then reject everything that he is and go back to being doers of what it reveals to be wickedness. Someone who disregarded everything that their tutor taught them after they left would be missing the whole point of a tutor.
In Galatians 3:26-29, every aspect of being in children of God, through faith, in Christ, and being children of Abraham and heirs to the promise is directly connected with being a doer of the Mosaic Law. In 1 John 3:4-10, those who are not doers of righteous works in obedience to the Law of Moses are not children of God. In Romans 3:31, our faith upholds the Law of Moses. In 1 John 2:6, those who are in Christ are obligated to walk in the same way that he walked. In John 8:39, Jesus said that if they were children of Abraham, then they would be doers of the same works as him.
