Who Is Israel?

What is the purpose of keeping the Sabbath?
Scripture tells us the purpose of keeping the Sabbath is so that we may know the true Elohim who sets us apart.  It is a sign which designates who the set-apart people are. 

Exo 31:13 "And you, speak to the children of Yisra’ĕl, saying, ‘My Sabbaths you are to guard, by all means, for it is a sign¹ between Me and you throughout your generations, to know that I, יהוה, am setting you apart. Footnote: 1The only sign of יהוה setting us apart, the only sign of the everlasting covenant, is His Sabbaths, one of them being the seventh day Sabbath. This is repeated in Ezek. 20:12 & 20.

Ok, I am taking it for granted that by now you realize the context says this is for Israel.  The next question we need to ask ourselves is "who is Israel"?  So, do we then expect that Jews who become Christians should observe Sunday?  No, rather it should be the other way around, if we are to be following Scripture.  Gentile believers according to Paul, become grafted into Israel.  They too become the spiritual seed of Abraham and thus a part of spiritual Israel.

Jer 11:16 "יהוה

has named you, ‘Green Olive Tree, Fair, of Goodly Fruit.’ With the noise of a great sound He has set it on fire, and its branches shall be broken.
Rom 11:17 And if some of the branches were broken off, and you, being a wild olive tree, have been grafted in among them, and came to share the root and fatness of the olive tree,

Rom 11:24 For if you were cut out of the olive tree which is wild by nature, and were grafted contrary to nature into a good olive tree, how much more shall these who are the natural branches, be grafted into their own olive tree?

Eph 2:11 Therefore remember that you, once gentiles¹ in the flesh, who are called ‘the uncircumcision’ by what is called ‘the circumcision’ made in the flesh by hands, Footnote: 11 Cor. 12:2.

Eph 2:12 that at that time you were without Messiah, excluded from the citizenship of Yisra’ĕl and strangers from the covenants of promise, having no expectation and without Elohim in the world.

Eph 2:13 But now in Messiah יהושע you who once were far off have been brought near by the blood of the Messiah.

Isa 56:6 "Also the sons of the foreigner who join themselves to יהוה, to serve Him, and to love the Name of יהוה, to be His servants, all who guard the Sabbath, and not profane it, and hold fast to My covenant –

Isa 56:7 them I shall bring to My set-apart mountain, and let them rejoice in My house of prayer. Their burnt offerings and their slaughterings are accepted on My altar, for My house is called a house of prayer for all the peoples
Yahshua told the Pharisees they were not of their father Abraham.  It confused them as they are the physical descendants of Abraham just as are the Arabs.  But Yahshua was not speaking of being physical descendants.  He was speaking of Spiritual descendants.  The sign that you are a spiritual descendant of Abraham and of his spiritual seed is that of keeping the Sabbath.  Being grafted into spiritual Israel, the wild branch into the tree planted by Yahweh Elohim.