- Matthew 5:18 [NASB20] "For truly I say to you, until heaven and earth pass away, not the smallest letter or stroke of a letter shall pass from the Law, until all is accomplished!"
While Jesus accomplished much through the cross, there is still the second coming and all that comes with it that is left to accomplish. Furthermore, heaven and earth have not passed away.
- John 19:30 [NASB20] Therefore when Jesus had received the sour wine, He said, "It is finished!" And He bowed His head and gave up His spirit.
In Titus 2:14, Jesus gave himself to redeem us form all lawlessness and to purify for himself people of his own possession who are zealous for doing good works, so becoming zealous for doing good works in obedience to God's law is the way to believe in what Jesus accomplished through the cross (Acts 21:20) while someone returning to the lawlessness that he gavIfe himself to redeem us from would be the way to reject what he accomplished.
- Were your fathers brought out of Egypt?
That fact that God saves His people out of bondage teaches us something that is true about the nature of who He is and we should live in a way that testifies about what we believe to be true about the nature of who God is regardless of whether we are physically descended from those that God brought out of Egypt. In John 8:39, Jesus said that if they were children of Abraham, then they would be doing the same works as him, so the way that the children of Abraham are multiplied in accordance with the promise is not through having many physical descendants, but through teaching people to do the same works as him in accordance with spending the Gospel.
- If not, then are you not following a covenant that God made with another people?
In Jeremiah 31:33, the New Covenant involves God putting the Mosaic Law in our minds and writing it on our hearts, so that is the way to live under the New Covenant.
- Was Paul wrong that such "circumcision" renders the cross of no value to you?
Either Paul only spoke against incorrect reasons for becoming circumcised or Galatians 5:2 means that Paul caused Christ to be of no value to Timothy when he had him circumcised and Christ is of no value to roughly 80% of the men in the US. In Acts 15:1, they were wanting to require all Gentiles to become circumcised in order to become saved, however, that was never the reason why God commanded circumcision, so the Jerusalem Council upheld the Mosaic Law by correctly ruling against requiring circumcision for an incorrect purpose. The Jerusalem Council did not have the authority to countermand God, so they should not be interpreted as trying to do that.
- Was the Holy Spirit wrong in advising the Jerusalem Council on the laws that Gentiles were to follow?
The Spirit has the role of leading in truth (John 16:13), the Spirit has the role of leading us to obey the Mosaic Law (Ezekiel 36:26-27), and the Mosaic Law is truth (Psalms 119:142). In John 16:8, the Spirit has the role of convicting us of sin and in Romans 3:20, it is by the Mosaic Law that we have knowledge of sin. In Romans 8:4-7, those who walk in the Spirit are contrasted with those who have minds set on the flesh who are enemies of God who refuse to submit to the Mosaic Law. In Galatians 5:19-23, everything listed as works of the flesh that are against the Spirit are also against the Mosaic Law while all of the fruits of the Spirit are aspects of God's nature that are in accordance with it. In Acts 5:32, the Spirit has been given to those who obey God. In Romans 2:25-29, the way to see that a Gentile has a circumcised heart is by observing their obedience to the Mosaic Law, which is the same way to tell for a Jew (Deuteronomy 30:6), and circumcision of the heart is a matter of the Spirit, which is in contrast with Acts 7:51-53, where those who have uncircumcised hearts resist the Spirit and do not obey the Mosaic Law. The Mosaic Law was given by God and the Spirit is God, so it is the Law of the Spirit.
If someone interprets Acts 15 as the Spirit countermanding the Father, then that would be a pretty strong indication that they were falsely claiming to be advised by the Spirit, which in part is why I don't think that interpretation is correct. Either the Acts 15:19-21 contains an exhaustive list of everything that would ever be required of a mature Gentile believer or it does not, so it is contradictory to treat it as being an exhaustive list to limit which laws Gentiles should follow while also treating it as being a non-exhaustive list by saying that there are obliviously other laws that Gentiles should follow, such as the greatest two commandment. Moreover, Jesus said in Matthew 22:36-40, that all of the other commandments hang on the greatest two, which means that they are all connected, so if you think that Gentiles should obey the greatest two commandments, then you should also think that Gentiles should obey the commandments than hang on them. In Acts 15:19-21, it was not intended as an exhaustive list for mature Gentile believers, but rather they stated that it was a list intended to not make things too difficult for new believers, which they excused by saying that they would continue to learn about how to obey Moses by hearing him taught every Sabbath in the synagogues.
[All this to say: I do not follow your logic of abrogating the clear words of the New Covenant to embrace an Old Covenant that has "passed away" ... been fulfilled and perfected ... swallowed up in the New.]
Where do you find fault with the reasoning that I used in the OP?