So Christ's sacrifice is not sufficient for them? They needed, and will need, to add works to His death on the cross? Even though Peter (one following the kingdom gospel) said,
Acts 15:11 KJV — But we believe that through the grace of the Lord Jesus Christ we shall be saved, even as they.
You're confusing the salvation of Israel with being saved by grace.
First, note that the Jerusalem Council (Acts 15) took place 14 years after Paul initially when to Jerusalem to see Peter (Galatians 1:18) and 17 years after Paul's conversion (which itself was about one year after Christ's ascension). Also note that Paul was able to explain his gospel to the leadership in Jerusalem, privately, before the council. 3 years, let alone 17 years, is plenty of time for Paul to have somewhat solidified his knowledge of what his dispensation means, and explaining things privately before they were discussed in public allowed him to "[communicate] to them that gospel which [Paul] preach[ed] among the Gentiles." He literally "laid out his case" to them, as the word used for "communicate" would imply.
This is why Peter could say "we shall be saved in the same manner as they."
Second, the salvation of Israel is when Jesus returns as King of the restored nation of Israel.
"Salvation is of the Jews." (John 4:22)
The law has ALWAYS been undergirded by grace, the "same manner" in Acts 15:11. The means are different, though.
Israel has always had a corporate relationship with God, their relationship with Him is "I (God) will be their God, and they shall be My people," and, "Where two or three are gathered in My name, there I will be with them also."
We in the body of Christ have individual relationships with God, 1-on-1, personal. Our relationship with Him is "you are the temple of God and the spirit of God Dwells in you," and, "members of the household of God," and, "sons of God through faith," and "as we have many members in one body, . . . we . . . are one body in Chist, and individually members of one another."
And Paul said,
Romans 11:6 KJV — And if by grace, then is it no more of works: otherwise grace is no more grace. But if it be of works, then is it no more grace: otherwise work is no more work.
Yes, if you have grace, then there is no need for works. The moment you add works (aka, the law) it's no longer grace, but works.
Not understanding this is why people like Hoping thinks that he is no longer capable of sinning, because in order to make his position rational, he has to redefine what works are.
Are you saying that Peter disagreed with James?
Peter and James taught the same gospel.
Paul taught something different than what they taught.
Peter, however, recognized that, while understanding that Grace was a part of both.
Hence: "a different gospel, which is not another." (Galatians 1:6-7)