We are not Jews by race because the world rejects God (Jesus) from the beginning. Nevertheless, From Antiquity till the end of the world, eSHABBAT has been for all willing men, according to Isaiah 56:1-8, that is, Jews and all Pagans. We understand in the aforementioned Isaiah passage that God did not exempt any converted Pagan from doing His will in any thing. In the passage, we cannot imagine a pagan convert in Antiquity bragging "Oh! No! I don't do Shabbat! I am not Jew!" Or anything else.
From the point of view of Isaiah 56 and Numbers 15:15-16, there had never been a difference between the Jews and the Pagans, except that the Pagans turn their back to God (Jesus) as the chosen people also did and that God (Jesus) would not force them to comply with His will.
Numbers 15:13-16 ASV
(13) All that are home-born shall do these things after this manner, in offering an offering made by fire, of a sweet savor unto Jehovah.
(14) And if a stranger sojourn with you, or whosoever may be among you throughout your generations, and will offer an offering made by fire, of a sweet savor unto Jehovah; as ye do, so he shall do.
(15) For the assembly, there shall be one statute for you, and for the stranger that sojourneth with you, a statute for ever throughout your generations: as ye are, so shall the sojourner be before Jehovah.
(16) One law and one ordinance shall be for you, and for the stranger that sojourneth with you.
So, it is foolishness that a converted to Christianity can elect to reject any doctrine in the Law and the Prophets because the person is not a Jew.